[lttng-ust.git] / libringbuffer / ring_buffer_frontend.c

/*
 * SPDX-License-Identifier: LGPL-2.1-only
 *
 * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * Ring buffer wait-free buffer synchronization. Producer-consumer and flight
 * recorder (overwrite) modes. See thesis:
 *
 * Desnoyers, Mathieu (2009), "Low-Impact Operating System Tracing", Ph.D.
 * dissertation, Ecole Polytechnique de Montreal.
 * http://www.lttng.org/pub/thesis/desnoyers-dissertation-2009-12.pdf
 *
 * - Algorithm presentation in Chapter 5:
 *     "Lockless Multi-Core High-Throughput Buffering".
 * - Algorithm formal verification in Section 8.6:
 *     "Formal verification of LTTng"
 *
 * Author:
 *      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * Inspired from LTT and RelayFS:
 *  Karim Yaghmour <karim@opersys.com>
 *  Tom Zanussi <zanussi@us.ibm.com>
 *  Bob Wisniewski <bob@watson.ibm.com>
 * And from K42 :
 *  Bob Wisniewski <bob@watson.ibm.com>
 *
 * Buffer reader semantic :
 *
 * - get_subbuf_size
 * while buffer is not finalized and empty
 *   - get_subbuf
 *     - if return value != 0, continue
 *   - splice one subbuffer worth of data to a pipe
 *   - splice the data from pipe to disk/network
 *   - put_subbuf
 */

#define _LGPL_SOURCE
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <time.h>
#include <stdbool.h>
#include <stdint.h>
#include <urcu/compiler.h>
#include <urcu/ref.h>
#include <urcu/tls-compat.h>
#include <poll.h>
#include <ust-helper.h>

#include <lttng/ust-utils.h>
#include <lttng/ringbuffer-context.h>

#include "smp.h"
#include "ringbuffer-config.h"
#include "vatomic.h"
#include "backend.h"
#include "frontend.h"
#include "shm.h"
#include "rb-init.h"
#include "../liblttng-ust/compat.h"     /* For ENODATA */

/* Print DBG() messages about events lost only every 1048576 hits */
#define DBG_PRINT_NR_LOST       (1UL << 20)

#define LTTNG_UST_RB_SIG_FLUSH          SIGRTMIN
#define LTTNG_UST_RB_SIG_READ           SIGRTMIN + 1
#define LTTNG_UST_RB_SIG_TEARDOWN       SIGRTMIN + 2
#define CLOCKID         CLOCK_MONOTONIC
#define LTTNG_UST_RING_BUFFER_GET_RETRY         10
#define LTTNG_UST_RING_BUFFER_RETRY_DELAY_MS    10
#define RETRY_DELAY_MS                          100     /* 100 ms. */

/*
 * Non-static to ensure the compiler does not optimize away the xor.
 */
uint8_t lttng_crash_magic_xor[] = RB_CRASH_DUMP_ABI_MAGIC_XOR;

/*
 * Use POSIX SHM: shm_open(3) and shm_unlink(3).
 * close(2) to close the fd returned by shm_open.
 * shm_unlink releases the shared memory object name.
 * ftruncate(2) sets the size of the memory object.
 * mmap/munmap maps the shared memory obj to a virtual address in the
 * calling proceess (should be done both in libust and consumer).
 * See shm_overview(7) for details.
 * Pass file descriptor returned by shm_open(3) to ltt-sessiond through
 * a UNIX socket.
 *
 * Since we don't need to access the object using its name, we can
 * immediately shm_unlink(3) it, and only keep the handle with its file
 * descriptor.
 */

/*
 * Internal structure representing offsets to use at a sub-buffer switch.
 */
struct switch_offsets {
        unsigned long begin, end, old;
        size_t pre_header_padding, size;
        unsigned int switch_new_start:1, switch_new_end:1, switch_old_start:1,
                     switch_old_end:1;
};

DEFINE_URCU_TLS(unsigned int, lib_ring_buffer_nesting);

/*
 * wakeup_fd_mutex protects wakeup fd use by timer from concurrent
 * close.
 */
static pthread_mutex_t wakeup_fd_mutex = PTHREAD_MUTEX_INITIALIZER;

static
void lib_ring_buffer_print_errors(struct lttng_ust_lib_ring_buffer_channel *chan,
                                struct lttng_ust_lib_ring_buffer *buf, int cpu,
                                struct lttng_ust_shm_handle *handle);

/*
 * Handle timer teardown race wrt memory free of private data by
 * ring buffer signals are handled by a single thread, which permits
 * a synchronization point between handling of each signal.
 * Protected by the lock within the structure.
 */
struct timer_signal_data {
        pthread_t tid;  /* thread id managing signals */
        int setup_done;
        int qs_done;
        pthread_mutex_t lock;
};

static struct timer_signal_data timer_signal = {
        .tid = 0,
        .setup_done = 0,
        .qs_done = 0,
        .lock = PTHREAD_MUTEX_INITIALIZER,
};

static bool lttng_ust_allow_blocking;

void lttng_ust_ringbuffer_set_allow_blocking(void)
{
        lttng_ust_allow_blocking = true;
}

/* Get blocking timeout, in ms */
static int lttng_ust_ringbuffer_get_timeout(struct lttng_ust_lib_ring_buffer_channel *chan)
{
        if (!lttng_ust_allow_blocking)
                return 0;
        return chan->u.s.blocking_timeout_ms;
}

/**
 * lib_ring_buffer_reset - Reset ring buffer to initial values.
 * @buf: Ring buffer.
 *
 * Effectively empty the ring buffer. Should be called when the buffer is not
 * used for writing. The ring buffer can be opened for reading, but the reader
 * should not be using the iterator concurrently with reset. The previous
 * current iterator record is reset.
 */
void lib_ring_buffer_reset(struct lttng_ust_lib_ring_buffer *buf,
                           struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_channel *chan;
        const struct lttng_ust_lib_ring_buffer_config *config;
        unsigned int i;

        chan = shmp(handle, buf->backend.chan);
        if (!chan)
                return;
        config = &chan->backend.config;
        /*
         * Reset iterator first. It will put the subbuffer if it currently holds
         * it.
         */
        v_set(config, &buf->offset, 0);
        for (i = 0; i < chan->backend.num_subbuf; i++) {
                struct commit_counters_hot *cc_hot;
                struct commit_counters_cold *cc_cold;
                uint64_t *ts_end;

                cc_hot = shmp_index(handle, buf->commit_hot, i);
                if (!cc_hot)
                        return;
                cc_cold = shmp_index(handle, buf->commit_cold, i);
                if (!cc_cold)
                        return;
                ts_end = shmp_index(handle, buf->ts_end, i);
                if (!ts_end)
                        return;
                v_set(config, &cc_hot->cc, 0);
                v_set(config, &cc_hot->seq, 0);
                v_set(config, &cc_cold->cc_sb, 0);
                *ts_end = 0;
        }
        uatomic_set(&buf->consumed, 0);
        uatomic_set(&buf->record_disabled, 0);
        v_set(config, &buf->last_tsc, 0);
        lib_ring_buffer_backend_reset(&buf->backend, handle);
        /* Don't reset number of active readers */
        v_set(config, &buf->records_lost_full, 0);
        v_set(config, &buf->records_lost_wrap, 0);
        v_set(config, &buf->records_lost_big, 0);
        v_set(config, &buf->records_count, 0);
        v_set(config, &buf->records_overrun, 0);
        buf->finalized = 0;
}

/**
 * channel_reset - Reset channel to initial values.
 * @chan: Channel.
 *
 * Effectively empty the channel. Should be called when the channel is not used
 * for writing. The channel can be opened for reading, but the reader should not
 * be using the iterator concurrently with reset. The previous current iterator
 * record is reset.
 */
void channel_reset(struct lttng_ust_lib_ring_buffer_channel *chan)
{
        /*
         * Reset iterators first. Will put the subbuffer if held for reading.
         */
        uatomic_set(&chan->record_disabled, 0);
        /* Don't reset commit_count_mask, still valid */
        channel_backend_reset(&chan->backend);
        /* Don't reset switch/read timer interval */
        /* Don't reset notifiers and notifier enable bits */
        /* Don't reset reader reference count */
}

static
void init_crash_abi(const struct lttng_ust_lib_ring_buffer_config *config,
                struct lttng_crash_abi *crash_abi,
                struct lttng_ust_lib_ring_buffer *buf,
                struct channel_backend *chanb,
                struct shm_object *shmobj,
                struct lttng_ust_shm_handle *handle)
{
        int i;

        for (i = 0; i < RB_CRASH_DUMP_ABI_MAGIC_LEN; i++)
                crash_abi->magic[i] = lttng_crash_magic_xor[i] ^ 0xFF;
        crash_abi->mmap_length = shmobj->memory_map_size;
        crash_abi->endian = RB_CRASH_ENDIAN;
        crash_abi->major = RB_CRASH_DUMP_ABI_MAJOR;
        crash_abi->minor = RB_CRASH_DUMP_ABI_MINOR;
        crash_abi->word_size = sizeof(unsigned long);
        crash_abi->layout_type = LTTNG_CRASH_TYPE_UST;

        /* Offset of fields */
        crash_abi->offset.prod_offset =
                (uint32_t) ((char *) &buf->offset - (char *) buf);
        crash_abi->offset.consumed_offset =
                (uint32_t) ((char *) &buf->consumed - (char *) buf);
        crash_abi->offset.commit_hot_array =
                (uint32_t) ((char *) shmp(handle, buf->commit_hot) - (char *) buf);
        crash_abi->offset.commit_hot_seq =
                offsetof(struct commit_counters_hot, seq);
        crash_abi->offset.buf_wsb_array =
                (uint32_t) ((char *) shmp(handle, buf->backend.buf_wsb) - (char *) buf);
        crash_abi->offset.buf_wsb_id =
                offsetof(struct lttng_ust_lib_ring_buffer_backend_subbuffer, id);
        crash_abi->offset.sb_array =
                (uint32_t) ((char *) shmp(handle, buf->backend.array) - (char *) buf);
        crash_abi->offset.sb_array_shmp_offset =
                offsetof(struct lttng_ust_lib_ring_buffer_backend_pages_shmp,
                        shmp._ref.offset);
        crash_abi->offset.sb_backend_p_offset =
                offsetof(struct lttng_ust_lib_ring_buffer_backend_pages,
                        p._ref.offset);

        /* Field length */
        crash_abi->length.prod_offset = sizeof(buf->offset);
        crash_abi->length.consumed_offset = sizeof(buf->consumed);
        crash_abi->length.commit_hot_seq =
                sizeof(((struct commit_counters_hot *) NULL)->seq);
        crash_abi->length.buf_wsb_id =
                sizeof(((struct lttng_ust_lib_ring_buffer_backend_subbuffer *) NULL)->id);
        crash_abi->length.sb_array_shmp_offset =
                sizeof(((struct lttng_ust_lib_ring_buffer_backend_pages_shmp *) NULL)->shmp._ref.offset);
        crash_abi->length.sb_backend_p_offset =
                sizeof(((struct lttng_ust_lib_ring_buffer_backend_pages *) NULL)->p._ref.offset);

        /* Array stride */
        crash_abi->stride.commit_hot_array =
                sizeof(struct commit_counters_hot);
        crash_abi->stride.buf_wsb_array =
                sizeof(struct lttng_ust_lib_ring_buffer_backend_subbuffer);
        crash_abi->stride.sb_array =
                sizeof(struct lttng_ust_lib_ring_buffer_backend_pages_shmp);

        /* Buffer constants */
        crash_abi->buf_size = chanb->buf_size;
        crash_abi->subbuf_size = chanb->subbuf_size;
        crash_abi->num_subbuf = chanb->num_subbuf;
        crash_abi->mode = (uint32_t) chanb->config.mode;

        if (config->cb.content_size_field) {
                size_t offset, length;

                config->cb.content_size_field(config, &offset, &length);
                crash_abi->offset.content_size = offset;
                crash_abi->length.content_size = length;
        } else {
                crash_abi->offset.content_size = 0;
                crash_abi->length.content_size = 0;
        }
        if (config->cb.packet_size_field) {
                size_t offset, length;

                config->cb.packet_size_field(config, &offset, &length);
                crash_abi->offset.packet_size = offset;
                crash_abi->length.packet_size = length;
        } else {
                crash_abi->offset.packet_size = 0;
                crash_abi->length.packet_size = 0;
        }
}

/*
 * Must be called under cpu hotplug protection.
 */
int lib_ring_buffer_create(struct lttng_ust_lib_ring_buffer *buf,
                           struct channel_backend *chanb, int cpu,
                           struct lttng_ust_shm_handle *handle,
                           struct shm_object *shmobj)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chanb->config;
        struct lttng_ust_lib_ring_buffer_channel *chan = caa_container_of(chanb,
                        struct lttng_ust_lib_ring_buffer_channel, backend);
        struct lttng_ust_lib_ring_buffer_backend_subbuffer *wsb;
        struct lttng_ust_lib_ring_buffer_channel *shmp_chan;
        struct commit_counters_hot *cc_hot;
        void *priv = channel_get_private_config(chan);
        size_t subbuf_header_size;
        uint64_t tsc;
        int ret;

        /* Test for cpu hotplug */
        if (buf->backend.allocated)
                return 0;

        align_shm(shmobj, __alignof__(struct commit_counters_hot));
        set_shmp(buf->commit_hot,
                 zalloc_shm(shmobj,
                        sizeof(struct commit_counters_hot) * chan->backend.num_subbuf));
        if (!shmp(handle, buf->commit_hot)) {
                return -ENOMEM;
        }

        align_shm(shmobj, __alignof__(struct commit_counters_cold));
        set_shmp(buf->commit_cold,
                 zalloc_shm(shmobj,
                        sizeof(struct commit_counters_cold) * chan->backend.num_subbuf));
        if (!shmp(handle, buf->commit_cold)) {
                ret = -ENOMEM;
                goto free_commit;
        }

        align_shm(shmobj, __alignof__(uint64_t));
        set_shmp(buf->ts_end,
                 zalloc_shm(shmobj,
                        sizeof(uint64_t) * chan->backend.num_subbuf));
        if (!shmp(handle, buf->ts_end)) {
                ret = -ENOMEM;
                goto free_commit_cold;
        }


        ret = lib_ring_buffer_backend_create(&buf->backend, &chan->backend,
                        cpu, handle, shmobj);
        if (ret) {
                goto free_init;
        }

        /*
         * Write the subbuffer header for first subbuffer so we know the total
         * duration of data gathering.
         */
        subbuf_header_size = config->cb.subbuffer_header_size();
        v_set(config, &buf->offset, subbuf_header_size);
        wsb = shmp_index(handle, buf->backend.buf_wsb, 0);
        if (!wsb) {
                ret = -EPERM;
                goto free_chanbuf;
        }
        subbuffer_id_clear_noref(config, &wsb->id);
        shmp_chan = shmp(handle, buf->backend.chan);
        if (!shmp_chan) {
                ret = -EPERM;
                goto free_chanbuf;
        }
        tsc = config->cb.ring_buffer_clock_read(shmp_chan);
        config->cb.buffer_begin(buf, tsc, 0, handle);
        cc_hot = shmp_index(handle, buf->commit_hot, 0);
        if (!cc_hot) {
                ret = -EPERM;
                goto free_chanbuf;
        }
        v_add(config, subbuf_header_size, &cc_hot->cc);
        v_add(config, subbuf_header_size, &cc_hot->seq);

        if (config->cb.buffer_create) {
                ret = config->cb.buffer_create(buf, priv, cpu, chanb->name, handle);
                if (ret)
                        goto free_chanbuf;
        }

        init_crash_abi(config, &buf->crash_abi, buf, chanb, shmobj, handle);

        buf->backend.allocated = 1;
        return 0;

        /* Error handling */
free_init:
        /* ts_end will be freed by shm teardown */
free_commit_cold:
        /* commit_cold will be freed by shm teardown */
free_commit:
        /* commit_hot will be freed by shm teardown */
free_chanbuf:
        return ret;
}

static
void lib_ring_buffer_channel_switch_timer(int sig, siginfo_t *si, void *uc)
{
        const struct lttng_ust_lib_ring_buffer_config *config;
        struct lttng_ust_shm_handle *handle;
        struct lttng_ust_lib_ring_buffer_channel *chan;
        int cpu;

        assert(CMM_LOAD_SHARED(timer_signal.tid) == pthread_self());

        chan = si->si_value.sival_ptr;
        handle = chan->handle;
        config = &chan->backend.config;

        DBG("Switch timer for channel %p\n", chan);

        /*
         * Only flush buffers periodically if readers are active.
         */
        pthread_mutex_lock(&wakeup_fd_mutex);
        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                for_each_possible_cpu(cpu) {
                        struct lttng_ust_lib_ring_buffer *buf =
                                shmp(handle, chan->backend.buf[cpu].shmp);

                        if (!buf)
                                goto end;
                        if (uatomic_read(&buf->active_readers))
                                lib_ring_buffer_switch_slow(buf, SWITCH_ACTIVE,
                                        chan->handle);
                }
        } else {
                struct lttng_ust_lib_ring_buffer *buf =
                        shmp(handle, chan->backend.buf[0].shmp);

                if (!buf)
                        goto end;
                if (uatomic_read(&buf->active_readers))
                        lib_ring_buffer_switch_slow(buf, SWITCH_ACTIVE,
                                chan->handle);
        }
end:
        pthread_mutex_unlock(&wakeup_fd_mutex);
        return;
}

static
int lib_ring_buffer_poll_deliver(const struct lttng_ust_lib_ring_buffer_config *config,
                                 struct lttng_ust_lib_ring_buffer *buf,
                                 struct lttng_ust_lib_ring_buffer_channel *chan,
                                 struct lttng_ust_shm_handle *handle)
{
        unsigned long consumed_old, consumed_idx, commit_count, write_offset;
        struct commit_counters_cold *cc_cold;

        consumed_old = uatomic_read(&buf->consumed);
        consumed_idx = subbuf_index(consumed_old, chan);
        cc_cold = shmp_index(handle, buf->commit_cold, consumed_idx);
        if (!cc_cold)
                return 0;
        commit_count = v_read(config, &cc_cold->cc_sb);
        /*
         * No memory barrier here, since we are only interested
         * in a statistically correct polling result. The next poll will
         * get the data is we are racing. The mb() that ensures correct
         * memory order is in get_subbuf.
         */
        write_offset = v_read(config, &buf->offset);

        /*
         * Check that the subbuffer we are trying to consume has been
         * already fully committed.
         */

        if (((commit_count - chan->backend.subbuf_size)
             & chan->commit_count_mask)
            - (buf_trunc(consumed_old, chan)
               >> chan->backend.num_subbuf_order)
            != 0)
                return 0;

        /*
         * Check that we are not about to read the same subbuffer in
         * which the writer head is.
         */
        if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed_old, chan)
            == 0)
                return 0;

        return 1;
}

static
void lib_ring_buffer_wakeup(struct lttng_ust_lib_ring_buffer *buf,
                struct lttng_ust_shm_handle *handle)
{
        int wakeup_fd = shm_get_wakeup_fd(handle, &buf->self._ref);
        sigset_t sigpipe_set, pending_set, old_set;
        int ret, sigpipe_was_pending = 0;

        if (wakeup_fd < 0)
                return;

        /*
         * Wake-up the other end by writing a null byte in the pipe
         * (non-blocking).  Important note: Because writing into the
         * pipe is non-blocking (and therefore we allow dropping wakeup
         * data, as long as there is wakeup data present in the pipe
         * buffer to wake up the consumer), the consumer should perform
         * the following sequence for waiting:
         * 1) empty the pipe (reads).
         * 2) check if there is data in the buffer.
         * 3) wait on the pipe (poll).
         *
         * Discard the SIGPIPE from write(), not disturbing any SIGPIPE
         * that might be already pending. If a bogus SIGPIPE is sent to
         * the entire process concurrently by a malicious user, it may
         * be simply discarded.
         */
        ret = sigemptyset(&pending_set);
        assert(!ret);
        /*
         * sigpending returns the mask of signals that are _both_
         * blocked for the thread _and_ pending for either the thread or
         * the entire process.
         */
        ret = sigpending(&pending_set);
        assert(!ret);
        sigpipe_was_pending = sigismember(&pending_set, SIGPIPE);
        /*
         * If sigpipe was pending, it means it was already blocked, so
         * no need to block it.
         */
        if (!sigpipe_was_pending) {
                ret = sigemptyset(&sigpipe_set);
                assert(!ret);
                ret = sigaddset(&sigpipe_set, SIGPIPE);
                assert(!ret);
                ret = pthread_sigmask(SIG_BLOCK, &sigpipe_set, &old_set);
                assert(!ret);
        }
        do {
                ret = write(wakeup_fd, "", 1);
        } while (ret == -1L && errno == EINTR);
        if (ret == -1L && errno == EPIPE && !sigpipe_was_pending) {
                struct timespec timeout = { 0, 0 };
                do {
                        ret = sigtimedwait(&sigpipe_set, NULL,
                                &timeout);
                } while (ret == -1L && errno == EINTR);
        }
        if (!sigpipe_was_pending) {
                ret = pthread_sigmask(SIG_SETMASK, &old_set, NULL);
                assert(!ret);
        }
}

static
void lib_ring_buffer_channel_do_read(struct lttng_ust_lib_ring_buffer_channel *chan)
{
        const struct lttng_ust_lib_ring_buffer_config *config;
        struct lttng_ust_shm_handle *handle;
        int cpu;

        handle = chan->handle;
        config = &chan->backend.config;

        /*
         * Only flush buffers periodically if readers are active.
         */
        pthread_mutex_lock(&wakeup_fd_mutex);
        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                for_each_possible_cpu(cpu) {
                        struct lttng_ust_lib_ring_buffer *buf =
                                shmp(handle, chan->backend.buf[cpu].shmp);

                        if (!buf)
                                goto end;
                        if (uatomic_read(&buf->active_readers)
                            && lib_ring_buffer_poll_deliver(config, buf,
                                        chan, handle)) {
                                lib_ring_buffer_wakeup(buf, handle);
                        }
                }
        } else {
                struct lttng_ust_lib_ring_buffer *buf =
                        shmp(handle, chan->backend.buf[0].shmp);

                if (!buf)
                        goto end;
                if (uatomic_read(&buf->active_readers)
                    && lib_ring_buffer_poll_deliver(config, buf,
                                chan, handle)) {
                        lib_ring_buffer_wakeup(buf, handle);
                }
        }
end:
        pthread_mutex_unlock(&wakeup_fd_mutex);
}

static
void lib_ring_buffer_channel_read_timer(int sig, siginfo_t *si, void *uc)
{
        struct lttng_ust_lib_ring_buffer_channel *chan;

        assert(CMM_LOAD_SHARED(timer_signal.tid) == pthread_self());
        chan = si->si_value.sival_ptr;
        DBG("Read timer for channel %p\n", chan);
        lib_ring_buffer_channel_do_read(chan);
        return;
}

static
void rb_setmask(sigset_t *mask)
{
        int ret;

        ret = sigemptyset(mask);
        if (ret) {
                PERROR("sigemptyset");
        }
        ret = sigaddset(mask, LTTNG_UST_RB_SIG_FLUSH);
        if (ret) {
                PERROR("sigaddset");
        }
        ret = sigaddset(mask, LTTNG_UST_RB_SIG_READ);
        if (ret) {
                PERROR("sigaddset");
        }
        ret = sigaddset(mask, LTTNG_UST_RB_SIG_TEARDOWN);
        if (ret) {
                PERROR("sigaddset");
        }
}

static
void *sig_thread(void *arg)
{
        sigset_t mask;
        siginfo_t info;
        int signr;

        /* Only self thread will receive signal mask. */
        rb_setmask(&mask);
        CMM_STORE_SHARED(timer_signal.tid, pthread_self());

        for (;;) {
                signr = sigwaitinfo(&mask, &info);
                if (signr == -1) {
                        if (errno != EINTR)
                                PERROR("sigwaitinfo");
                        continue;
                }
                if (signr == LTTNG_UST_RB_SIG_FLUSH) {
                        lib_ring_buffer_channel_switch_timer(info.si_signo,
                                        &info, NULL);
                } else if (signr == LTTNG_UST_RB_SIG_READ) {
                        lib_ring_buffer_channel_read_timer(info.si_signo,
                                        &info, NULL);
                } else if (signr == LTTNG_UST_RB_SIG_TEARDOWN) {
                        cmm_smp_mb();
                        CMM_STORE_SHARED(timer_signal.qs_done, 1);
                        cmm_smp_mb();
                } else {
                        ERR("Unexptected signal %d\n", info.si_signo);
                }
        }
        return NULL;
}

/*
 * Ensure only a single thread listens on the timer signal.
 */
static
void lib_ring_buffer_setup_timer_thread(void)
{
        pthread_t thread;
        int ret;

        pthread_mutex_lock(&timer_signal.lock);
        if (timer_signal.setup_done)
                goto end;

        ret = pthread_create(&thread, NULL, &sig_thread, NULL);
        if (ret) {
                errno = ret;
                PERROR("pthread_create");
        }
        ret = pthread_detach(thread);
        if (ret) {
                errno = ret;
                PERROR("pthread_detach");
        }
        timer_signal.setup_done = 1;
end:
        pthread_mutex_unlock(&timer_signal.lock);
}

/*
 * Wait for signal-handling thread quiescent state.
 */
static
void lib_ring_buffer_wait_signal_thread_qs(unsigned int signr)
{
        sigset_t pending_set;
        int ret;

        /*
         * We need to be the only thread interacting with the thread
         * that manages signals for teardown synchronization.
         */
        pthread_mutex_lock(&timer_signal.lock);

        /*
         * Ensure we don't have any signal queued for this channel.
         */
        for (;;) {
                ret = sigemptyset(&pending_set);
                if (ret == -1) {
                        PERROR("sigemptyset");
                }
                ret = sigpending(&pending_set);
                if (ret == -1) {
                        PERROR("sigpending");
                }
                if (!sigismember(&pending_set, signr))
                        break;
                caa_cpu_relax();
        }

        /*
         * From this point, no new signal handler will be fired that
         * would try to access "chan". However, we still need to wait
         * for any currently executing handler to complete.
         */
        cmm_smp_mb();
        CMM_STORE_SHARED(timer_signal.qs_done, 0);
        cmm_smp_mb();

        /*
         * Kill with LTTNG_UST_RB_SIG_TEARDOWN, so signal management
         * thread wakes up.
         */
        kill(getpid(), LTTNG_UST_RB_SIG_TEARDOWN);

        while (!CMM_LOAD_SHARED(timer_signal.qs_done))
                caa_cpu_relax();
        cmm_smp_mb();

        pthread_mutex_unlock(&timer_signal.lock);
}

static
void lib_ring_buffer_channel_switch_timer_start(struct lttng_ust_lib_ring_buffer_channel *chan)
{
        struct sigevent sev;
        struct itimerspec its;
        int ret;

        if (!chan->switch_timer_interval || chan->switch_timer_enabled)
                return;

        chan->switch_timer_enabled = 1;

        lib_ring_buffer_setup_timer_thread();

        memset(&sev, 0, sizeof(sev));
        sev.sigev_notify = SIGEV_SIGNAL;
        sev.sigev_signo = LTTNG_UST_RB_SIG_FLUSH;
        sev.sigev_value.sival_ptr = chan;
        ret = timer_create(CLOCKID, &sev, &chan->switch_timer);
        if (ret == -1) {
                PERROR("timer_create");
        }

        its.it_value.tv_sec = chan->switch_timer_interval / 1000000;
        its.it_value.tv_nsec = (chan->switch_timer_interval % 1000000) * 1000;
        its.it_interval.tv_sec = its.it_value.tv_sec;
        its.it_interval.tv_nsec = its.it_value.tv_nsec;

        ret = timer_settime(chan->switch_timer, 0, &its, NULL);
        if (ret == -1) {
                PERROR("timer_settime");
        }
}

static
void lib_ring_buffer_channel_switch_timer_stop(struct lttng_ust_lib_ring_buffer_channel *chan)
{
        int ret;

        if (!chan->switch_timer_interval || !chan->switch_timer_enabled)
                return;

        ret = timer_delete(chan->switch_timer);
        if (ret == -1) {
                PERROR("timer_delete");
        }

        lib_ring_buffer_wait_signal_thread_qs(LTTNG_UST_RB_SIG_FLUSH);

        chan->switch_timer = 0;
        chan->switch_timer_enabled = 0;
}

static
void lib_ring_buffer_channel_read_timer_start(struct lttng_ust_lib_ring_buffer_channel *chan)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        struct sigevent sev;
        struct itimerspec its;
        int ret;

        if (config->wakeup != RING_BUFFER_WAKEUP_BY_TIMER
                        || !chan->read_timer_interval || chan->read_timer_enabled)
                return;

        chan->read_timer_enabled = 1;

        lib_ring_buffer_setup_timer_thread();

        sev.sigev_notify = SIGEV_SIGNAL;
        sev.sigev_signo = LTTNG_UST_RB_SIG_READ;
        sev.sigev_value.sival_ptr = chan;
        ret = timer_create(CLOCKID, &sev, &chan->read_timer);
        if (ret == -1) {
                PERROR("timer_create");
        }

        its.it_value.tv_sec = chan->read_timer_interval / 1000000;
        its.it_value.tv_nsec = (chan->read_timer_interval % 1000000) * 1000;
        its.it_interval.tv_sec = its.it_value.tv_sec;
        its.it_interval.tv_nsec = its.it_value.tv_nsec;

        ret = timer_settime(chan->read_timer, 0, &its, NULL);
        if (ret == -1) {
                PERROR("timer_settime");
        }
}

static
void lib_ring_buffer_channel_read_timer_stop(struct lttng_ust_lib_ring_buffer_channel *chan)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        int ret;

        if (config->wakeup != RING_BUFFER_WAKEUP_BY_TIMER
                        || !chan->read_timer_interval || !chan->read_timer_enabled)
                return;

        ret = timer_delete(chan->read_timer);
        if (ret == -1) {
                PERROR("timer_delete");
        }

        /*
         * do one more check to catch data that has been written in the last
         * timer period.
         */
        lib_ring_buffer_channel_do_read(chan);

        lib_ring_buffer_wait_signal_thread_qs(LTTNG_UST_RB_SIG_READ);

        chan->read_timer = 0;
        chan->read_timer_enabled = 0;
}

static void channel_unregister_notifiers(struct lttng_ust_lib_ring_buffer_channel *chan,
                           struct lttng_ust_shm_handle *handle)
{
        lib_ring_buffer_channel_switch_timer_stop(chan);
        lib_ring_buffer_channel_read_timer_stop(chan);
}

static void channel_print_errors(struct lttng_ust_lib_ring_buffer_channel *chan,
                struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config =
                        &chan->backend.config;
        int cpu;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                for_each_possible_cpu(cpu) {
                        struct lttng_ust_lib_ring_buffer *buf =
                                shmp(handle, chan->backend.buf[cpu].shmp);
                        if (buf)
                                lib_ring_buffer_print_errors(chan, buf, cpu, handle);
                }
        } else {
                struct lttng_ust_lib_ring_buffer *buf =
                        shmp(handle, chan->backend.buf[0].shmp);

                if (buf)
                        lib_ring_buffer_print_errors(chan, buf, -1, handle);
        }
}

static void channel_free(struct lttng_ust_lib_ring_buffer_channel *chan,
                struct lttng_ust_shm_handle *handle,
                int consumer)
{
        channel_backend_free(&chan->backend, handle);
        /* chan is freed by shm teardown */
        shm_object_table_destroy(handle->table, consumer);
        free(handle);
}

/**
 * channel_create - Create channel.
 * @config: ring buffer instance configuration
 * @name: name of the channel
 * @priv_data_align: alignment, in bytes, of the private data area. (config)
 * @priv_data_size: length, in bytes, of the private data area. (config)
 * @priv_data_init: initialization data for private data. (config)
 * @priv: local private data (memory owner by caller)
 * @buf_addr: pointer the the beginning of the preallocated buffer contiguous
 *            address mapping. It is used only by RING_BUFFER_STATIC
 *            configuration. It can be set to NULL for other backends.
 * @subbuf_size: subbuffer size
 * @num_subbuf: number of subbuffers
 * @switch_timer_interval: Time interval (in us) to fill sub-buffers with
 *                         padding to let readers get those sub-buffers.
 *                         Used for live streaming.
 * @read_timer_interval: Time interval (in us) to wake up pending readers.
 * @stream_fds: array of stream file descriptors.
 * @nr_stream_fds: number of file descriptors in array.
 *
 * Holds cpu hotplug.
 * Returns NULL on failure.
 */
struct lttng_ust_shm_handle *channel_create(const struct lttng_ust_lib_ring_buffer_config *config,
                   const char *name,
                   size_t priv_data_align,
                   size_t priv_data_size,
                   void *priv_data_init,
                   void *priv,
                   void *buf_addr, size_t subbuf_size,
                   size_t num_subbuf, unsigned int switch_timer_interval,
                   unsigned int read_timer_interval,
                   const int *stream_fds, int nr_stream_fds,
                   int64_t blocking_timeout)
{
        int ret;
        size_t shmsize, chansize;
        struct lttng_ust_lib_ring_buffer_channel *chan;
        struct lttng_ust_shm_handle *handle;
        struct shm_object *shmobj;
        unsigned int nr_streams;
        int64_t blocking_timeout_ms;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
                nr_streams = num_possible_cpus();
        else
                nr_streams = 1;

        if (nr_stream_fds != nr_streams)
                return NULL;

        if (blocking_timeout < -1) {
                return NULL;
        }
        /* usec to msec */
        if (blocking_timeout == -1) {
                blocking_timeout_ms = -1;
        } else {
                blocking_timeout_ms = blocking_timeout / 1000;
                if (blocking_timeout_ms != (int32_t) blocking_timeout_ms) {
                        return NULL;
                }
        }

        if (lib_ring_buffer_check_config(config, switch_timer_interval,
                                         read_timer_interval))
                return NULL;

        handle = zmalloc(sizeof(struct lttng_ust_shm_handle));
        if (!handle)
                return NULL;

        /* Allocate table for channel + per-cpu buffers */
        handle->table = shm_object_table_create(1 + num_possible_cpus());
        if (!handle->table)
                goto error_table_alloc;

        /* Calculate the shm allocation layout */
        shmsize = sizeof(struct lttng_ust_lib_ring_buffer_channel);
        shmsize += lttng_ust_offset_align(shmsize, __alignof__(struct lttng_ust_lib_ring_buffer_shmp));
        shmsize += sizeof(struct lttng_ust_lib_ring_buffer_shmp) * nr_streams;
        chansize = shmsize;
        if (priv_data_align)
                shmsize += lttng_ust_offset_align(shmsize, priv_data_align);
        shmsize += priv_data_size;

        /* Allocate normal memory for channel (not shared) */
        shmobj = shm_object_table_alloc(handle->table, shmsize, SHM_OBJECT_MEM,
                        -1, -1);
        if (!shmobj)
                goto error_append;
        /* struct lttng_ust_lib_ring_buffer_channel is at object 0, offset 0 (hardcoded) */
        set_shmp(handle->chan, zalloc_shm(shmobj, chansize));
        assert(handle->chan._ref.index == 0);
        assert(handle->chan._ref.offset == 0);
        chan = shmp(handle, handle->chan);
        if (!chan)
                goto error_append;
        chan->nr_streams = nr_streams;

        /* space for private data */
        if (priv_data_size) {
                void *priv_config;

                DECLARE_SHMP(void, priv_data_alloc);

                align_shm(shmobj, priv_data_align);
                chan->priv_data_offset = shmobj->allocated_len;
                set_shmp(priv_data_alloc, zalloc_shm(shmobj, priv_data_size));
                if (!shmp(handle, priv_data_alloc))
                        goto error_append;
                priv_config = channel_get_private_config(chan);
                memcpy(priv_config, priv_data_init, priv_data_size);
        } else {
                chan->priv_data_offset = -1;
        }

        chan->u.s.blocking_timeout_ms = (int32_t) blocking_timeout_ms;

        channel_set_private(chan, priv);

        ret = channel_backend_init(&chan->backend, name, config,
                                   subbuf_size, num_subbuf, handle,
                                   stream_fds);
        if (ret)
                goto error_backend_init;

        chan->handle = handle;
        chan->commit_count_mask = (~0UL >> chan->backend.num_subbuf_order);

        chan->switch_timer_interval = switch_timer_interval;
        chan->read_timer_interval = read_timer_interval;
        lib_ring_buffer_channel_switch_timer_start(chan);
        lib_ring_buffer_channel_read_timer_start(chan);

        return handle;

error_backend_init:
error_append:
        shm_object_table_destroy(handle->table, 1);
error_table_alloc:
        free(handle);
        return NULL;
}

struct lttng_ust_shm_handle *channel_handle_create(void *data,
                                        uint64_t memory_map_size,
                                        int wakeup_fd)
{
        struct lttng_ust_shm_handle *handle;
        struct shm_object *object;

        handle = zmalloc(sizeof(struct lttng_ust_shm_handle));
        if (!handle)
                return NULL;

        /* Allocate table for channel + per-cpu buffers */
        handle->table = shm_object_table_create(1 + num_possible_cpus());
        if (!handle->table)
                goto error_table_alloc;
        /* Add channel object */
        object = shm_object_table_append_mem(handle->table, data,
                        memory_map_size, wakeup_fd);
        if (!object)
                goto error_table_object;
        /* struct lttng_ust_lib_ring_buffer_channel is at object 0, offset 0 (hardcoded) */
        handle->chan._ref.index = 0;
        handle->chan._ref.offset = 0;
        return handle;

error_table_object:
        shm_object_table_destroy(handle->table, 0);
error_table_alloc:
        free(handle);
        return NULL;
}

int channel_handle_add_stream(struct lttng_ust_shm_handle *handle,
                int shm_fd, int wakeup_fd, uint32_t stream_nr,
                uint64_t memory_map_size)
{
        struct shm_object *object;

        /* Add stream object */
        object = shm_object_table_append_shm(handle->table,
                        shm_fd, wakeup_fd, stream_nr,
                        memory_map_size);
        if (!object)
                return -EINVAL;
        return 0;
}

unsigned int channel_handle_get_nr_streams(struct lttng_ust_shm_handle *handle)
{
        assert(handle->table);
        return handle->table->allocated_len - 1;
}

static
void channel_release(struct lttng_ust_lib_ring_buffer_channel *chan, struct lttng_ust_shm_handle *handle,
                int consumer)
{
        channel_free(chan, handle, consumer);
}

/**
 * channel_destroy - Finalize, wait for q.s. and destroy channel.
 * @chan: channel to destroy
 *
 * Holds cpu hotplug.
 * Call "destroy" callback, finalize channels, decrement the channel
 * reference count. Note that when readers have completed data
 * consumption of finalized channels, get_subbuf() will return -ENODATA.
 * They should release their handle at that point.
 */
void channel_destroy(struct lttng_ust_lib_ring_buffer_channel *chan, struct lttng_ust_shm_handle *handle,
                int consumer)
{
        if (consumer) {
                /*
                 * Note: the consumer takes care of finalizing and
                 * switching the buffers.
                 */
                channel_unregister_notifiers(chan, handle);
                /*
                 * The consumer prints errors.
                 */
                channel_print_errors(chan, handle);
        }

        /*
         * sessiond/consumer are keeping a reference on the shm file
         * descriptor directly. No need to refcount.
         */
        channel_release(chan, handle, consumer);
        return;
}

struct lttng_ust_lib_ring_buffer *channel_get_ring_buffer(
                                        const struct lttng_ust_lib_ring_buffer_config *config,
                                        struct lttng_ust_lib_ring_buffer_channel *chan, int cpu,
                                        struct lttng_ust_shm_handle *handle,
                                        int *shm_fd, int *wait_fd,
                                        int *wakeup_fd,
                                        uint64_t *memory_map_size)
{
        struct shm_ref *ref;

        if (config->alloc == RING_BUFFER_ALLOC_GLOBAL) {
                cpu = 0;
        } else {
                if (cpu >= num_possible_cpus())
                        return NULL;
        }
        ref = &chan->backend.buf[cpu].shmp._ref;
        *shm_fd = shm_get_shm_fd(handle, ref);
        *wait_fd = shm_get_wait_fd(handle, ref);
        *wakeup_fd = shm_get_wakeup_fd(handle, ref);
        if (shm_get_shm_size(handle, ref, memory_map_size))
                return NULL;
        return shmp(handle, chan->backend.buf[cpu].shmp);
}

int ring_buffer_channel_close_wait_fd(const struct lttng_ust_lib_ring_buffer_config *config,
                        struct lttng_ust_lib_ring_buffer_channel *chan,
                        struct lttng_ust_shm_handle *handle)
{
        struct shm_ref *ref;

        ref = &handle->chan._ref;
        return shm_close_wait_fd(handle, ref);
}

int ring_buffer_channel_close_wakeup_fd(const struct lttng_ust_lib_ring_buffer_config *config,
                        struct lttng_ust_lib_ring_buffer_channel *chan,
                        struct lttng_ust_shm_handle *handle)
{
        struct shm_ref *ref;

        ref = &handle->chan._ref;
        return shm_close_wakeup_fd(handle, ref);
}

int ring_buffer_stream_close_wait_fd(const struct lttng_ust_lib_ring_buffer_config *config,
                        struct lttng_ust_lib_ring_buffer_channel *chan,
                        struct lttng_ust_shm_handle *handle,
                        int cpu)
{
        struct shm_ref *ref;

        if (config->alloc == RING_BUFFER_ALLOC_GLOBAL) {
                cpu = 0;
        } else {
                if (cpu >= num_possible_cpus())
                        return -EINVAL;
        }
        ref = &chan->backend.buf[cpu].shmp._ref;
        return shm_close_wait_fd(handle, ref);
}

int ring_buffer_stream_close_wakeup_fd(const struct lttng_ust_lib_ring_buffer_config *config,
                        struct lttng_ust_lib_ring_buffer_channel *chan,
                        struct lttng_ust_shm_handle *handle,
                        int cpu)
{
        struct shm_ref *ref;
        int ret;

        if (config->alloc == RING_BUFFER_ALLOC_GLOBAL) {
                cpu = 0;
        } else {
                if (cpu >= num_possible_cpus())
                        return -EINVAL;
        }
        ref = &chan->backend.buf[cpu].shmp._ref;
        pthread_mutex_lock(&wakeup_fd_mutex);
        ret = shm_close_wakeup_fd(handle, ref);
        pthread_mutex_unlock(&wakeup_fd_mutex);
        return ret;
}

int lib_ring_buffer_open_read(struct lttng_ust_lib_ring_buffer *buf,
                              struct lttng_ust_shm_handle *handle)
{
        if (uatomic_cmpxchg(&buf->active_readers, 0, 1) != 0)
                return -EBUSY;
        cmm_smp_mb();
        return 0;
}

void lib_ring_buffer_release_read(struct lttng_ust_lib_ring_buffer *buf,
                                  struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_channel *chan = shmp(handle, buf->backend.chan);

        if (!chan)
                return;
        CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1);
        cmm_smp_mb();
        uatomic_dec(&buf->active_readers);
}

/**
 * lib_ring_buffer_snapshot - save subbuffer position snapshot (for read)
 * @buf: ring buffer
 * @consumed: consumed count indicating the position where to read
 * @produced: produced count, indicates position when to stop reading
 *
 * Returns -ENODATA if buffer is finalized, -EAGAIN if there is currently no
 * data to read at consumed position, or 0 if the get operation succeeds.
 */

int lib_ring_buffer_snapshot(struct lttng_ust_lib_ring_buffer *buf,
                             unsigned long *consumed, unsigned long *produced,
                             struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_channel *chan;
        const struct lttng_ust_lib_ring_buffer_config *config;
        unsigned long consumed_cur, write_offset;
        int finalized;

        chan = shmp(handle, buf->backend.chan);
        if (!chan)
                return -EPERM;
        config = &chan->backend.config;
        finalized = CMM_ACCESS_ONCE(buf->finalized);
        /*
         * Read finalized before counters.
         */
        cmm_smp_rmb();
        consumed_cur = uatomic_read(&buf->consumed);
        /*
         * No need to issue a memory barrier between consumed count read and
         * write offset read, because consumed count can only change
         * concurrently in overwrite mode, and we keep a sequence counter
         * identifier derived from the write offset to check we are getting
         * the same sub-buffer we are expecting (the sub-buffers are atomically
         * "tagged" upon writes, tags are checked upon read).
         */
        write_offset = v_read(config, &buf->offset);

        /*
         * Check that we are not about to read the same subbuffer in
         * which the writer head is.
         */
        if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed_cur, chan)
            == 0)
                goto nodata;

        *consumed = consumed_cur;
        *produced = subbuf_trunc(write_offset, chan);

        return 0;

nodata:
        /*
         * The memory barriers __wait_event()/wake_up_interruptible() take care
         * of "raw_spin_is_locked" memory ordering.
         */
        if (finalized)
                return -ENODATA;
        else
                return -EAGAIN;
}

/**
 * Performs the same function as lib_ring_buffer_snapshot(), but the positions
 * are saved regardless of whether the consumed and produced positions are
 * in the same subbuffer.
 * @buf: ring buffer
 * @consumed: consumed byte count indicating the last position read
 * @produced: produced byte count indicating the last position written
 *
 * This function is meant to provide information on the exact producer and
 * consumer positions without regard for the "snapshot" feature.
 */
int lib_ring_buffer_snapshot_sample_positions(
                             struct lttng_ust_lib_ring_buffer *buf,
                             unsigned long *consumed, unsigned long *produced,
                             struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_channel *chan;
        const struct lttng_ust_lib_ring_buffer_config *config;

        chan = shmp(handle, buf->backend.chan);
        if (!chan)
                return -EPERM;
        config = &chan->backend.config;
        cmm_smp_rmb();
        *consumed = uatomic_read(&buf->consumed);
        /*
         * No need to issue a memory barrier between consumed count read and
         * write offset read, because consumed count can only change
         * concurrently in overwrite mode, and we keep a sequence counter
         * identifier derived from the write offset to check we are getting
         * the same sub-buffer we are expecting (the sub-buffers are atomically
         * "tagged" upon writes, tags are checked upon read).
         */
        *produced = v_read(config, &buf->offset);
        return 0;
}

/**
 * lib_ring_buffer_move_consumer - move consumed counter forward
 * @buf: ring buffer
 * @consumed_new: new consumed count value
 */
void lib_ring_buffer_move_consumer(struct lttng_ust_lib_ring_buffer *buf,
                                   unsigned long consumed_new,
                                   struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_backend *bufb = &buf->backend;
        struct lttng_ust_lib_ring_buffer_channel *chan;
        unsigned long consumed;

        chan = shmp(handle, bufb->chan);
        if (!chan)
                return;
        CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1);

        /*
         * Only push the consumed value forward.
         * If the consumed cmpxchg fails, this is because we have been pushed by
         * the writer in flight recorder mode.
         */
        consumed = uatomic_read(&buf->consumed);
        while ((long) consumed - (long) consumed_new < 0)
                consumed = uatomic_cmpxchg(&buf->consumed, consumed,
                                           consumed_new);
}

/**
 * lib_ring_buffer_get_subbuf - get exclusive access to subbuffer for reading
 * @buf: ring buffer
 * @consumed: consumed count indicating the position where to read
 *
 * Returns -ENODATA if buffer is finalized, -EAGAIN if there is currently no
 * data to read at consumed position, or 0 if the get operation succeeds.
 */
int lib_ring_buffer_get_subbuf(struct lttng_ust_lib_ring_buffer *buf,
                               unsigned long consumed,
                               struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_channel *chan;
        const struct lttng_ust_lib_ring_buffer_config *config;
        unsigned long consumed_cur, consumed_idx, commit_count, write_offset;
        int ret, finalized, nr_retry = LTTNG_UST_RING_BUFFER_GET_RETRY;
        struct commit_counters_cold *cc_cold;

        chan = shmp(handle, buf->backend.chan);
        if (!chan)
                return -EPERM;
        config = &chan->backend.config;
retry:
        finalized = CMM_ACCESS_ONCE(buf->finalized);
        /*
         * Read finalized before counters.
         */
        cmm_smp_rmb();
        consumed_cur = uatomic_read(&buf->consumed);
        consumed_idx = subbuf_index(consumed, chan);
        cc_cold = shmp_index(handle, buf->commit_cold, consumed_idx);
        if (!cc_cold)
                return -EPERM;
        commit_count = v_read(config, &cc_cold->cc_sb);
        /*
         * Make sure we read the commit count before reading the buffer
         * data and the write offset. Correct consumed offset ordering
         * wrt commit count is insured by the use of cmpxchg to update
         * the consumed offset.
         */
        /*
         * Local rmb to match the remote wmb to read the commit count
         * before the buffer data and the write offset.
         */
        cmm_smp_rmb();

        write_offset = v_read(config, &buf->offset);

        /*
         * Check that the buffer we are getting is after or at consumed_cur
         * position.
         */
        if ((long) subbuf_trunc(consumed, chan)
            - (long) subbuf_trunc(consumed_cur, chan) < 0)
                goto nodata;

        /*
         * Check that the subbuffer we are trying to consume has been
         * already fully committed. There are a few causes that can make
         * this unavailability situation occur:
         *
         * Temporary (short-term) situation:
         * - Application is running on a different CPU, between reserve
         *   and commit ring buffer operations,
         * - Application is preempted between reserve and commit ring
         *   buffer operations,
         *
         * Long-term situation:
         * - Application is stopped (SIGSTOP) between reserve and commit
         *   ring buffer operations. Could eventually be resumed by
         *   SIGCONT.
         * - Application is killed (SIGTERM, SIGINT, SIGKILL) between
         *   reserve and commit ring buffer operation.
         *
         * From a consumer perspective, handling short-term
         * unavailability situations is performed by retrying a few
         * times after a delay. Handling long-term unavailability
         * situations is handled by failing to get the sub-buffer.
         *
         * In all of those situations, if the application is taking a
         * long time to perform its commit after ring buffer space
         * reservation, we can end up in a situation where the producer
         * will fill the ring buffer and try to write into the same
         * sub-buffer again (which has a missing commit). This is
         * handled by the producer in the sub-buffer switch handling
         * code of the reserve routine by detecting unbalanced
         * reserve/commit counters and discarding all further events
         * until the situation is resolved in those situations. Two
         * scenarios can occur:
         *
         * 1) The application causing the reserve/commit counters to be
         *    unbalanced has been terminated. In this situation, all
         *    further events will be discarded in the buffers, and no
         *    further buffer data will be readable by the consumer
         *    daemon. Tearing down the UST tracing session and starting
         *    anew is a work-around for those situations. Note that this
         *    only affects per-UID tracing. In per-PID tracing, the
         *    application vanishes with the termination, and therefore
         *    no more data needs to be written to the buffers.
         * 2) The application causing the unbalance has been delayed for
         *    a long time, but will eventually try to increment the
         *    commit counter after eventually writing to the sub-buffer.
         *    This situation can cause events to be discarded until the
         *    application resumes its operations.
         */
        if (((commit_count - chan->backend.subbuf_size)
             & chan->commit_count_mask)
            - (buf_trunc(consumed, chan)
               >> chan->backend.num_subbuf_order)
            != 0) {
                if (nr_retry-- > 0) {
                        if (nr_retry <= (LTTNG_UST_RING_BUFFER_GET_RETRY >> 1))
                                (void) poll(NULL, 0, LTTNG_UST_RING_BUFFER_RETRY_DELAY_MS);
                        goto retry;
                } else {
                        goto nodata;
                }
        }

        /*
         * Check that we are not about to read the same subbuffer in
         * which the writer head is.
         */
        if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed, chan)
            == 0)
                goto nodata;

        /*
         * Failure to get the subbuffer causes a busy-loop retry without going
         * to a wait queue. These are caused by short-lived race windows where
         * the writer is getting access to a subbuffer we were trying to get
         * access to. Also checks that the "consumed" buffer count we are
         * looking for matches the one contained in the subbuffer id.
         *
         * The short-lived race window described here can be affected by
         * application signals and preemption, thus requiring to bound
         * the loop to a maximum number of retry.
         */
        ret = update_read_sb_index(config, &buf->backend, &chan->backend,
                                   consumed_idx, buf_trunc_val(consumed, chan),
                                   handle);
        if (ret) {
                if (nr_retry-- > 0) {
                        if (nr_retry <= (LTTNG_UST_RING_BUFFER_GET_RETRY >> 1))
                                (void) poll(NULL, 0, LTTNG_UST_RING_BUFFER_RETRY_DELAY_MS);
                        goto retry;
                } else {
                        goto nodata;
                }
        }
        subbuffer_id_clear_noref(config, &buf->backend.buf_rsb.id);

        buf->get_subbuf_consumed = consumed;
        buf->get_subbuf = 1;

        return 0;

nodata:
        /*
         * The memory barriers __wait_event()/wake_up_interruptible() take care
         * of "raw_spin_is_locked" memory ordering.
         */
        if (finalized)
                return -ENODATA;
        else
                return -EAGAIN;
}

/**
 * lib_ring_buffer_put_subbuf - release exclusive subbuffer access
 * @buf: ring buffer
 */
void lib_ring_buffer_put_subbuf(struct lttng_ust_lib_ring_buffer *buf,
                                struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_backend *bufb = &buf->backend;
        struct lttng_ust_lib_ring_buffer_channel *chan;
        const struct lttng_ust_lib_ring_buffer_config *config;
        unsigned long sb_bindex, consumed_idx, consumed;
        struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
        struct lttng_ust_lib_ring_buffer_backend_pages *backend_pages;

        chan = shmp(handle, bufb->chan);
        if (!chan)
                return;
        config = &chan->backend.config;
        CHAN_WARN_ON(chan, uatomic_read(&buf->active_readers) != 1);

        if (!buf->get_subbuf) {
                /*
                 * Reader puts a subbuffer it did not get.
                 */
                CHAN_WARN_ON(chan, 1);
                return;
        }
        consumed = buf->get_subbuf_consumed;
        buf->get_subbuf = 0;

        /*
         * Clear the records_unread counter. (overruns counter)
         * Can still be non-zero if a file reader simply grabbed the data
         * without using iterators.
         * Can be below zero if an iterator is used on a snapshot more than
         * once.
         */
        sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
        rpages = shmp_index(handle, bufb->array, sb_bindex);
        if (!rpages)
                return;
        backend_pages = shmp(handle, rpages->shmp);
        if (!backend_pages)
                return;
        v_add(config, v_read(config, &backend_pages->records_unread),
                        &bufb->records_read);
        v_set(config, &backend_pages->records_unread, 0);
        CHAN_WARN_ON(chan, config->mode == RING_BUFFER_OVERWRITE
                     && subbuffer_id_is_noref(config, bufb->buf_rsb.id));
        subbuffer_id_set_noref(config, &bufb->buf_rsb.id);

        /*
         * Exchange the reader subbuffer with the one we put in its place in the
         * writer subbuffer table. Expect the original consumed count. If
         * update_read_sb_index fails, this is because the writer updated the
         * subbuffer concurrently. We should therefore keep the subbuffer we
         * currently have: it has become invalid to try reading this sub-buffer
         * consumed count value anyway.
         */
        consumed_idx = subbuf_index(consumed, chan);
        update_read_sb_index(config, &buf->backend, &chan->backend,
                             consumed_idx, buf_trunc_val(consumed, chan),
                             handle);
        /*
         * update_read_sb_index return value ignored. Don't exchange sub-buffer
         * if the writer concurrently updated it.
         */
}

/*
 * cons_offset is an iterator on all subbuffer offsets between the reader
 * position and the writer position. (inclusive)
 */
static
void lib_ring_buffer_print_subbuffer_errors(struct lttng_ust_lib_ring_buffer *buf,
                                            struct lttng_ust_lib_ring_buffer_channel *chan,
                                            unsigned long cons_offset,
                                            int cpu,
                                            struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long cons_idx, commit_count, commit_count_sb;
        struct commit_counters_hot *cc_hot;
        struct commit_counters_cold *cc_cold;

        cons_idx = subbuf_index(cons_offset, chan);
        cc_hot = shmp_index(handle, buf->commit_hot, cons_idx);
        if (!cc_hot)
                return;
        cc_cold = shmp_index(handle, buf->commit_cold, cons_idx);
        if (!cc_cold)
                return;
        commit_count = v_read(config, &cc_hot->cc);
        commit_count_sb = v_read(config, &cc_cold->cc_sb);

        if (subbuf_offset(commit_count, chan) != 0)
                DBG("ring buffer %s, cpu %d: "
                       "commit count in subbuffer %lu,\n"
                       "expecting multiples of %lu bytes\n"
                       "  [ %lu bytes committed, %lu bytes reader-visible ]\n",
                       chan->backend.name, cpu, cons_idx,
                       chan->backend.subbuf_size,
                       commit_count, commit_count_sb);

        DBG("ring buffer: %s, cpu %d: %lu bytes committed\n",
               chan->backend.name, cpu, commit_count);
}

static
void lib_ring_buffer_print_buffer_errors(struct lttng_ust_lib_ring_buffer *buf,
                                         struct lttng_ust_lib_ring_buffer_channel *chan,
                                         int cpu, struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long write_offset, cons_offset;

        /*
         * No need to order commit_count, write_offset and cons_offset reads
         * because we execute at teardown when no more writer nor reader
         * references are left.
         */
        write_offset = v_read(config, &buf->offset);
        cons_offset = uatomic_read(&buf->consumed);
        if (write_offset != cons_offset)
                DBG("ring buffer %s, cpu %d: "
                       "non-consumed data\n"
                       "  [ %lu bytes written, %lu bytes read ]\n",
                       chan->backend.name, cpu, write_offset, cons_offset);

        for (cons_offset = uatomic_read(&buf->consumed);
             (long) (subbuf_trunc((unsigned long) v_read(config, &buf->offset),
                                  chan)
                     - cons_offset) > 0;
             cons_offset = subbuf_align(cons_offset, chan))
                lib_ring_buffer_print_subbuffer_errors(buf, chan, cons_offset,
                                                       cpu, handle);
}

static
void lib_ring_buffer_print_errors(struct lttng_ust_lib_ring_buffer_channel *chan,
                                struct lttng_ust_lib_ring_buffer *buf, int cpu,
                                struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;

        if (!strcmp(chan->backend.name, "relay-metadata-mmap")) {
                DBG("ring buffer %s: %lu records written, "
                        "%lu records overrun\n",
                        chan->backend.name,
                        v_read(config, &buf->records_count),
                        v_read(config, &buf->records_overrun));
        } else {
                DBG("ring buffer %s, cpu %d: %lu records written, "
                        "%lu records overrun\n",
                        chan->backend.name, cpu,
                        v_read(config, &buf->records_count),
                        v_read(config, &buf->records_overrun));

                if (v_read(config, &buf->records_lost_full)
                    || v_read(config, &buf->records_lost_wrap)
                    || v_read(config, &buf->records_lost_big))
                        DBG("ring buffer %s, cpu %d: records were lost. Caused by:\n"
                                "  [ %lu buffer full, %lu nest buffer wrap-around, "
                                "%lu event too big ]\n",
                                chan->backend.name, cpu,
                                v_read(config, &buf->records_lost_full),
                                v_read(config, &buf->records_lost_wrap),
                                v_read(config, &buf->records_lost_big));
        }
        lib_ring_buffer_print_buffer_errors(buf, chan, cpu, handle);
}

/*
 * lib_ring_buffer_switch_old_start: Populate old subbuffer header.
 *
 * Only executed by SWITCH_FLUSH, which can be issued while tracing is
 * active or at buffer finalization (destroy).
 */
static
void lib_ring_buffer_switch_old_start(struct lttng_ust_lib_ring_buffer *buf,
                                      struct lttng_ust_lib_ring_buffer_channel *chan,
                                      struct switch_offsets *offsets,
                                      uint64_t tsc,
                                      struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long oldidx = subbuf_index(offsets->old, chan);
        unsigned long commit_count;
        struct commit_counters_hot *cc_hot;

        config->cb.buffer_begin(buf, tsc, oldidx, handle);

        /*
         * Order all writes to buffer before the commit count update that will
         * determine that the subbuffer is full.
         */
        cmm_smp_wmb();
        cc_hot = shmp_index(handle, buf->commit_hot, oldidx);
        if (!cc_hot)
                return;
        v_add(config, config->cb.subbuffer_header_size(),
              &cc_hot->cc);
        commit_count = v_read(config, &cc_hot->cc);
        /* Check if the written buffer has to be delivered */
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->old,
                                      commit_count, oldidx, handle, tsc);
        lib_ring_buffer_write_commit_counter(config, buf, chan,
                        offsets->old + config->cb.subbuffer_header_size(),
                        commit_count, handle, cc_hot);
}

/*
 * lib_ring_buffer_switch_old_end: switch old subbuffer
 *
 * Note : offset_old should never be 0 here. It is ok, because we never perform
 * buffer switch on an empty subbuffer in SWITCH_ACTIVE mode. The caller
 * increments the offset_old value when doing a SWITCH_FLUSH on an empty
 * subbuffer.
 */
static
void lib_ring_buffer_switch_old_end(struct lttng_ust_lib_ring_buffer *buf,
                                    struct lttng_ust_lib_ring_buffer_channel *chan,
                                    struct switch_offsets *offsets,
                                    uint64_t tsc,
                                    struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long oldidx = subbuf_index(offsets->old - 1, chan);
        unsigned long commit_count, padding_size, data_size;
        struct commit_counters_hot *cc_hot;
        uint64_t *ts_end;

        data_size = subbuf_offset(offsets->old - 1, chan) + 1;
        padding_size = chan->backend.subbuf_size - data_size;
        subbuffer_set_data_size(config, &buf->backend, oldidx, data_size,
                                handle);

        ts_end = shmp_index(handle, buf->ts_end, oldidx);
        if (!ts_end)
                return;
        /*
         * This is the last space reservation in that sub-buffer before
         * it gets delivered. This provides exclusive access to write to
         * this sub-buffer's ts_end. There are also no concurrent
         * readers of that ts_end because delivery of that sub-buffer is
         * postponed until the commit counter is incremented for the
         * current space reservation.
         */
        *ts_end = tsc;

        /*
         * Order all writes to buffer and store to ts_end before the commit
         * count update that will determine that the subbuffer is full.
         */
        cmm_smp_wmb();
        cc_hot = shmp_index(handle, buf->commit_hot, oldidx);
        if (!cc_hot)
                return;
        v_add(config, padding_size, &cc_hot->cc);
        commit_count = v_read(config, &cc_hot->cc);
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->old - 1,
                                      commit_count, oldidx, handle, tsc);
        lib_ring_buffer_write_commit_counter(config, buf, chan,
                        offsets->old + padding_size, commit_count, handle,
                        cc_hot);
}

/*
 * lib_ring_buffer_switch_new_start: Populate new subbuffer.
 *
 * This code can be executed unordered : writers may already have written to the
 * sub-buffer before this code gets executed, caution.  The commit makes sure
 * that this code is executed before the deliver of this sub-buffer.
 */
static
void lib_ring_buffer_switch_new_start(struct lttng_ust_lib_ring_buffer *buf,
                                      struct lttng_ust_lib_ring_buffer_channel *chan,
                                      struct switch_offsets *offsets,
                                      uint64_t tsc,
                                      struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long beginidx = subbuf_index(offsets->begin, chan);
        unsigned long commit_count;
        struct commit_counters_hot *cc_hot;

        config->cb.buffer_begin(buf, tsc, beginidx, handle);

        /*
         * Order all writes to buffer before the commit count update that will
         * determine that the subbuffer is full.
         */
        cmm_smp_wmb();
        cc_hot = shmp_index(handle, buf->commit_hot, beginidx);
        if (!cc_hot)
                return;
        v_add(config, config->cb.subbuffer_header_size(), &cc_hot->cc);
        commit_count = v_read(config, &cc_hot->cc);
        /* Check if the written buffer has to be delivered */
        lib_ring_buffer_check_deliver(config, buf, chan, offsets->begin,
                                      commit_count, beginidx, handle, tsc);
        lib_ring_buffer_write_commit_counter(config, buf, chan,
                        offsets->begin + config->cb.subbuffer_header_size(),
                        commit_count, handle, cc_hot);
}

/*
 * lib_ring_buffer_switch_new_end: finish switching current subbuffer
 *
 * Calls subbuffer_set_data_size() to set the data size of the current
 * sub-buffer. We do not need to perform check_deliver nor commit here,
 * since this task will be done by the "commit" of the event for which
 * we are currently doing the space reservation.
 */
static
void lib_ring_buffer_switch_new_end(struct lttng_ust_lib_ring_buffer *buf,
                                    struct lttng_ust_lib_ring_buffer_channel *chan,
                                    struct switch_offsets *offsets,
                                    uint64_t tsc,
                                    struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long endidx, data_size;
        uint64_t *ts_end;

        endidx = subbuf_index(offsets->end - 1, chan);
        data_size = subbuf_offset(offsets->end - 1, chan) + 1;
        subbuffer_set_data_size(config, &buf->backend, endidx, data_size,
                                handle);
        ts_end = shmp_index(handle, buf->ts_end, endidx);
        if (!ts_end)
                return;
        /*
         * This is the last space reservation in that sub-buffer before
         * it gets delivered. This provides exclusive access to write to
         * this sub-buffer's ts_end. There are also no concurrent
         * readers of that ts_end because delivery of that sub-buffer is
         * postponed until the commit counter is incremented for the
         * current space reservation.
         */
        *ts_end = tsc;
}

/*
 * Returns :
 * 0 if ok
 * !0 if execution must be aborted.
 */
static
int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
                                    struct lttng_ust_lib_ring_buffer *buf,
                                    struct lttng_ust_lib_ring_buffer_channel *chan,
                                    struct switch_offsets *offsets,
                                    uint64_t *tsc,
                                    struct lttng_ust_shm_handle *handle)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        unsigned long off, reserve_commit_diff;

        offsets->begin = v_read(config, &buf->offset);
        offsets->old = offsets->begin;
        offsets->switch_old_start = 0;
        off = subbuf_offset(offsets->begin, chan);

        *tsc = config->cb.ring_buffer_clock_read(chan);

        /*
         * Ensure we flush the header of an empty subbuffer when doing the
         * finalize (SWITCH_FLUSH). This ensures that we end up knowing the
         * total data gathering duration even if there were no records saved
         * after the last buffer switch.
         * In SWITCH_ACTIVE mode, switch the buffer when it contains events.
         * SWITCH_ACTIVE only flushes the current subbuffer, dealing with end of
         * subbuffer header as appropriate.
         * The next record that reserves space will be responsible for
         * populating the following subbuffer header. We choose not to populate
         * the next subbuffer header here because we want to be able to use
         * SWITCH_ACTIVE for periodical buffer flush, which must
         * guarantee that all the buffer content (records and header
         * timestamps) are visible to the reader. This is required for
         * quiescence guarantees for the fusion merge.
         */
        if (mode != SWITCH_FLUSH && !off)
                return -1;      /* we do not have to switch : buffer is empty */

        if (caa_unlikely(off == 0)) {
                unsigned long sb_index, commit_count;
                struct commit_counters_cold *cc_cold;

                /*
                 * We are performing a SWITCH_FLUSH. There may be concurrent
                 * writes into the buffer if e.g. invoked while performing a
                 * snapshot on an active trace.
                 *
                 * If the client does not save any header information
                 * (sub-buffer header size == 0), don't switch empty subbuffer
                 * on finalize, because it is invalid to deliver a completely
                 * empty subbuffer.
                 */
                if (!config->cb.subbuffer_header_size())
                        return -1;

                /* Test new buffer integrity */
                sb_index = subbuf_index(offsets->begin, chan);
                cc_cold = shmp_index(handle, buf->commit_cold, sb_index);
                if (!cc_cold)
                        return -1;
                commit_count = v_read(config, &cc_cold->cc_sb);
                reserve_commit_diff =
                  (buf_trunc(offsets->begin, chan)
                   >> chan->backend.num_subbuf_order)
                  - (commit_count & chan->commit_count_mask);
                if (caa_likely(reserve_commit_diff == 0)) {
                        /* Next subbuffer not being written to. */
                        if (caa_unlikely(config->mode != RING_BUFFER_OVERWRITE &&
                                subbuf_trunc(offsets->begin, chan)
                                 - subbuf_trunc((unsigned long)
                                     uatomic_read(&buf->consumed), chan)
                                >= chan->backend.buf_size)) {
                                /*
                                 * We do not overwrite non consumed buffers
                                 * and we are full : don't switch.
                                 */
                                return -1;
                        } else {
                                /*
                                 * Next subbuffer not being written to, and we
                                 * are either in overwrite mode or the buffer is
                                 * not full. It's safe to write in this new
                                 * subbuffer.
                                 */
                        }
                } else {
                        /*
                         * Next subbuffer reserve offset does not match the
                         * commit offset. Don't perform switch in
                         * producer-consumer and overwrite mode.  Caused by
                         * either a writer OOPS or too many nested writes over a
                         * reserve/commit pair.
                         */
                        return -1;
                }

                /*
                 * Need to write the subbuffer start header on finalize.
                 */
                offsets->switch_old_start = 1;
        }
        offsets->begin = subbuf_align(offsets->begin, chan);
        /* Note: old points to the next subbuf at offset 0 */
        offsets->end = offsets->begin;
        return 0;
}

/*
 * Force a sub-buffer switch. This operation is completely reentrant : can be
 * called while tracing is active with absolutely no lock held.
 *
 * For RING_BUFFER_SYNC_PER_CPU ring buffers, as a v_cmpxchg is used for
 * some atomic operations, this function must be called from the CPU
 * which owns the buffer for a ACTIVE flush. However, for
 * RING_BUFFER_SYNC_GLOBAL ring buffers, this function can be called
 * from any CPU.
 */
void lib_ring_buffer_switch_slow(struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
                                 struct lttng_ust_shm_handle *handle)
{
        struct lttng_ust_lib_ring_buffer_channel *chan;
        const struct lttng_ust_lib_ring_buffer_config *config;
        struct switch_offsets offsets;
        unsigned long oldidx;
        uint64_t tsc;

        chan = shmp(handle, buf->backend.chan);
        if (!chan)
                return;
        config = &chan->backend.config;

        offsets.size = 0;

        /*
         * Perform retryable operations.
         */
        do {
                if (lib_ring_buffer_try_switch_slow(mode, buf, chan, &offsets,
                                                    &tsc, handle))
                        return; /* Switch not needed */
        } while (v_cmpxchg(config, &buf->offset, offsets.old, offsets.end)
                 != offsets.old);

        /*
         * Atomically update last_tsc. This update races against concurrent
         * atomic updates, but the race will always cause supplementary full TSC
         * records, never the opposite (missing a full TSC record when it would
         * be needed).
         */
        save_last_tsc(config, buf, tsc);

        /*
         * Push the reader if necessary
         */
        lib_ring_buffer_reserve_push_reader(buf, chan, offsets.old);

        oldidx = subbuf_index(offsets.old, chan);
        lib_ring_buffer_clear_noref(config, &buf->backend, oldidx, handle);

        /*
         * May need to populate header start on SWITCH_FLUSH.
         */
        if (offsets.switch_old_start) {
                lib_ring_buffer_switch_old_start(buf, chan, &offsets, tsc, handle);
                offsets.old += config->cb.subbuffer_header_size();
        }

        /*
         * Switch old subbuffer.
         */
        lib_ring_buffer_switch_old_end(buf, chan, &offsets, tsc, handle);
}

static
bool handle_blocking_retry(int *timeout_left_ms)
{
        int timeout = *timeout_left_ms, delay;

        if (caa_likely(!timeout))
                return false;   /* Do not retry, discard event. */
        if (timeout < 0)        /* Wait forever. */
                delay = RETRY_DELAY_MS;
        else
                delay = min_t(int, timeout, RETRY_DELAY_MS);
        (void) poll(NULL, 0, delay);
        if (timeout > 0)
                *timeout_left_ms -= delay;
        return true;    /* Retry. */
}

/*
 * Returns :
 * 0 if ok
 * -ENOSPC if event size is too large for packet.
 * -ENOBUFS if there is currently not enough space in buffer for the event.
 * -EIO if data cannot be written into the buffer for any other reason.
 */
static
int lib_ring_buffer_try_reserve_slow(struct lttng_ust_lib_ring_buffer *buf,
                                     struct lttng_ust_lib_ring_buffer_channel *chan,
                                     struct switch_offsets *offsets,
                                     struct lttng_ust_lib_ring_buffer_ctx *ctx,
                                     void *client_ctx)
{
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        struct lttng_ust_shm_handle *handle = ctx->chan->handle;
        unsigned long reserve_commit_diff, offset_cmp;
        int timeout_left_ms = lttng_ust_ringbuffer_get_timeout(chan);

retry:
        offsets->begin = offset_cmp = v_read(config, &buf->offset);
        offsets->old = offsets->begin;
        offsets->switch_new_start = 0;
        offsets->switch_new_end = 0;
        offsets->switch_old_end = 0;
        offsets->pre_header_padding = 0;

        ctx->tsc = config->cb.ring_buffer_clock_read(chan);
        if ((int64_t) ctx->tsc == -EIO)
                return -EIO;

        if (last_tsc_overflow(config, buf, ctx->tsc))
                ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;

        if (caa_unlikely(subbuf_offset(offsets->begin, ctx->chan) == 0)) {
                offsets->switch_new_start = 1;          /* For offsets->begin */
        } else {
                offsets->size = config->cb.record_header_size(config, chan,
                                                offsets->begin,
                                                &offsets->pre_header_padding,
                                                ctx, client_ctx);
                offsets->size +=
                        lttng_ust_lib_ring_buffer_align(offsets->begin + offsets->size,
                                              ctx->largest_align)
                        + ctx->data_size;
                if (caa_unlikely(subbuf_offset(offsets->begin, chan) +
                             offsets->size > chan->backend.subbuf_size)) {
                        offsets->switch_old_end = 1;    /* For offsets->old */
                        offsets->switch_new_start = 1;  /* For offsets->begin */
                }
        }
        if (caa_unlikely(offsets->switch_new_start)) {
                unsigned long sb_index, commit_count;
                struct commit_counters_cold *cc_cold;

                /*
                 * We are typically not filling the previous buffer completely.
                 */
                if (caa_likely(offsets->switch_old_end))
                        offsets->begin = subbuf_align(offsets->begin, chan);
                offsets->begin = offsets->begin
                                 + config->cb.subbuffer_header_size();
                /* Test new buffer integrity */
                sb_index = subbuf_index(offsets->begin, chan);
                /*
                 * Read buf->offset before buf->commit_cold[sb_index].cc_sb.
                 * lib_ring_buffer_check_deliver() has the matching
                 * memory barriers required around commit_cold cc_sb
                 * updates to ensure reserve and commit counter updates
                 * are not seen reordered when updated by another CPU.
                 */
                cmm_smp_rmb();
                cc_cold = shmp_index(handle, buf->commit_cold, sb_index);
                if (!cc_cold)
                        return -1;
                commit_count = v_read(config, &cc_cold->cc_sb);
                /* Read buf->commit_cold[sb_index].cc_sb before buf->offset. */
                cmm_smp_rmb();
                if (caa_unlikely(offset_cmp != v_read(config, &buf->offset))) {
                        /*
                         * The reserve counter have been concurrently updated
                         * while we read the commit counter. This means the
                         * commit counter we read might not match buf->offset
                         * due to concurrent update. We therefore need to retry.
                         */
                        goto retry;
                }
                reserve_commit_diff =
                  (buf_trunc(offsets->begin, chan)
                   >> chan->backend.num_subbuf_order)
                  - (commit_count & chan->commit_count_mask);
                if (caa_likely(reserve_commit_diff == 0)) {
                        /* Next subbuffer not being written to. */
                        if (caa_unlikely(config->mode != RING_BUFFER_OVERWRITE &&
                                subbuf_trunc(offsets->begin, chan)
                                 - subbuf_trunc((unsigned long)
                                     uatomic_read(&buf->consumed), chan)
                                >= chan->backend.buf_size)) {
                                unsigned long nr_lost;

                                if (handle_blocking_retry(&timeout_left_ms))
                                        goto retry;

                                /*
                                 * We do not overwrite non consumed buffers
                                 * and we are full : record is lost.
                                 */
                                nr_lost = v_read(config, &buf->records_lost_full);
                                v_inc(config, &buf->records_lost_full);
                                if ((nr_lost & (DBG_PRINT_NR_LOST - 1)) == 0) {
                                        DBG("%lu or more records lost in (%s:%d) (buffer full)\n",
                                                nr_lost + 1, chan->backend.name,
                                                buf->backend.cpu);
                                }
                                return -ENOBUFS;
                        } else {
                                /*
                                 * Next subbuffer not being written to, and we
                                 * are either in overwrite mode or the buffer is
                                 * not full. It's safe to write in this new
                                 * subbuffer.
                                 */
                        }
                } else {
                        unsigned long nr_lost;

                        /*
                         * Next subbuffer reserve offset does not match the
                         * commit offset, and this did not involve update to the
                         * reserve counter. Drop record in producer-consumer and
                         * overwrite mode. Caused by either a writer OOPS or too
                         * many nested writes over a reserve/commit pair.
                         */
                        nr_lost = v_read(config, &buf->records_lost_wrap);
                        v_inc(config, &buf->records_lost_wrap);
                        if ((nr_lost & (DBG_PRINT_NR_LOST - 1)) == 0) {
                                DBG("%lu or more records lost in (%s:%d) (wrap-around)\n",
                                        nr_lost + 1, chan->backend.name,
                                        buf->backend.cpu);
                        }
                        return -EIO;
                }
                offsets->size =
                        config->cb.record_header_size(config, chan,
                                                offsets->begin,
                                                &offsets->pre_header_padding,
                                                ctx, client_ctx);
                offsets->size +=
                        lttng_ust_lib_ring_buffer_align(offsets->begin + offsets->size,
                                              ctx->largest_align)
                        + ctx->data_size;
                if (caa_unlikely(subbuf_offset(offsets->begin, chan)
                             + offsets->size > chan->backend.subbuf_size)) {
                        unsigned long nr_lost;

                        /*
                         * Record too big for subbuffers, report error, don't
                         * complete the sub-buffer switch.
                         */
                        nr_lost = v_read(config, &buf->records_lost_big);
                        v_inc(config, &buf->records_lost_big);
                        if ((nr_lost & (DBG_PRINT_NR_LOST - 1)) == 0) {
                                DBG("%lu or more records lost in (%s:%d) record size "
                                        " of %zu bytes is too large for buffer\n",
                                        nr_lost + 1, chan->backend.name,
                                        buf->backend.cpu, offsets->size);
                        }
                        return -ENOSPC;
                } else {
                        /*
                         * We just made a successful buffer switch and the
                         * record fits in the new subbuffer. Let's write.
                         */
                }
        } else {
                /*
                 * Record fits in the current buffer and we are not on a switch
                 * boundary. It's safe to write.
                 */
        }
        offsets->end = offsets->begin + offsets->size;

        if (caa_unlikely(subbuf_offset(offsets->end, chan) == 0)) {
                /*
                 * The offset_end will fall at the very beginning of the next
                 * subbuffer.
                 */
                offsets->switch_new_end = 1;    /* For offsets->begin */
        }
        return 0;
}

/**
 * lib_ring_buffer_reserve_slow - Atomic slot reservation in a buffer.
 * @ctx: ring buffer context.
 *
 * Return : -NOBUFS if not enough space, -ENOSPC if event size too large,
 * -EIO for other errors, else returns 0.
 * It will take care of sub-buffer switching.
 */
int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx,
                void *client_ctx)
{
        struct lttng_ust_lib_ring_buffer_channel *chan = ctx->chan;
        struct lttng_ust_shm_handle *handle = ctx->chan->handle;
        const struct lttng_ust_lib_ring_buffer_config *config = &chan->backend.config;
        struct lttng_ust_lib_ring_buffer *buf;
        struct switch_offsets offsets;
        int ret;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
                buf = shmp(handle, chan->backend.buf[ctx->reserve_cpu].shmp);
        else
                buf = shmp(handle, chan->backend.buf[0].shmp);
        if (!buf)
                return -EIO;
        ctx->buf = buf;

        offsets.size = 0;

        do {
                ret = lib_ring_buffer_try_reserve_slow(buf, chan, &offsets,
                                                       ctx, client_ctx);
                if (caa_unlikely(ret))
                        return ret;
        } while (caa_unlikely(v_cmpxchg(config, &buf->offset, offsets.old,
                                    offsets.end)
                          != offsets.old));

        /*
         * Atomically update last_tsc. This update races against concurrent
         * atomic updates, but the race will always cause supplementary full TSC
         * records, never the opposite (missing a full TSC record when it would
         * be needed).
         */
        save_last_tsc(config, buf, ctx->tsc);

        /*
         * Push the reader if necessary
         */
        lib_ring_buffer_reserve_push_reader(buf, chan, offsets.end - 1);

        /*
         * Clear noref flag for this subbuffer.
         */
        lib_ring_buffer_clear_noref(config, &buf->backend,
                                    subbuf_index(offsets.end - 1, chan),
                                    handle);

        /*
         * Switch old subbuffer if needed.
         */
        if (caa_unlikely(offsets.switch_old_end)) {
                lib_ring_buffer_clear_noref(config, &buf->backend,
                                            subbuf_index(offsets.old - 1, chan),
                                            handle);
                lib_ring_buffer_switch_old_end(buf, chan, &offsets, ctx->tsc, handle);
        }

        /*
         * Populate new subbuffer.
         */
        if (caa_unlikely(offsets.switch_new_start))
                lib_ring_buffer_switch_new_start(buf, chan, &offsets, ctx->tsc, handle);

        if (caa_unlikely(offsets.switch_new_end))
                lib_ring_buffer_switch_new_end(buf, chan, &offsets, ctx->tsc, handle);

        ctx->slot_size = offsets.size;
        ctx->pre_offset = offsets.begin;
        ctx->buf_offset = offsets.begin + offsets.pre_header_padding;
        return 0;
}

static
void lib_ring_buffer_vmcore_check_deliver(const struct lttng_ust_lib_ring_buffer_config *config,
                                          struct lttng_ust_lib_ring_buffer *buf,
                                          unsigned long commit_count,
                                          unsigned long idx,
                                          struct lttng_ust_shm_handle *handle)
{
        struct commit_counters_hot *cc_hot;

        if (config->oops != RING_BUFFER_OOPS_CONSISTENCY)
                return;
        cc_hot = shmp_index(handle, buf->commit_hot, idx);
        if (!cc_hot)
                return;
        v_set(config, &cc_hot->seq, commit_count);
}

/*
 * The ring buffer can count events recorded and overwritten per buffer,
 * but it is disabled by default due to its performance overhead.
 */
#ifdef LTTNG_RING_BUFFER_COUNT_EVENTS
static
void deliver_count_events(const struct lttng_ust_lib_ring_buffer_config *config,
                struct lttng_ust_lib_ring_buffer *buf,
                unsigned long idx,
                struct lttng_ust_shm_handle *handle)
{
        v_add(config, subbuffer_get_records_count(config,
                        &buf->backend, idx, handle),
                &buf->records_count);
        v_add(config, subbuffer_count_records_overrun(config,
                        &buf->backend, idx, handle),
                &buf->records_overrun);
}
#else /* LTTNG_RING_BUFFER_COUNT_EVENTS */
static
void deliver_count_events(const struct lttng_ust_lib_ring_buffer_config *config,
                struct lttng_ust_lib_ring_buffer *buf,
                unsigned long idx,
                struct lttng_ust_shm_handle *handle)
{
}
#endif /* #else LTTNG_RING_BUFFER_COUNT_EVENTS */

void lib_ring_buffer_check_deliver_slow(const struct lttng_ust_lib_ring_buffer_config *config,
                                   struct lttng_ust_lib_ring_buffer *buf,
                                   struct lttng_ust_lib_ring_buffer_channel *chan,
                                   unsigned long offset,
                                   unsigned long commit_count,
                                   unsigned long idx,
                                   struct lttng_ust_shm_handle *handle,
                                   uint64_t tsc)
{
        unsigned long old_commit_count = commit_count
                                         - chan->backend.subbuf_size;
        struct commit_counters_cold *cc_cold;

        /*
         * If we succeeded at updating cc_sb below, we are the subbuffer
         * writer delivering the subbuffer. Deals with concurrent
         * updates of the "cc" value without adding a add_return atomic
         * operation to the fast path.
         *
         * We are doing the delivery in two steps:
         * - First, we cmpxchg() cc_sb to the new value
         *   old_commit_count + 1. This ensures that we are the only
         *   subbuffer user successfully filling the subbuffer, but we
         *   do _not_ set the cc_sb value to "commit_count" yet.
         *   Therefore, other writers that would wrap around the ring
         *   buffer and try to start writing to our subbuffer would
         *   have to drop records, because it would appear as
         *   non-filled.
         *   We therefore have exclusive access to the subbuffer control
         *   structures.  This mutual exclusion with other writers is
         *   crucially important to perform record overruns count in
         *   flight recorder mode locklessly.
         * - When we are ready to release the subbuffer (either for
         *   reading or for overrun by other writers), we simply set the
         *   cc_sb value to "commit_count" and perform delivery.
         *
         * The subbuffer size is least 2 bytes (minimum size: 1 page).
         * This guarantees that old_commit_count + 1 != commit_count.
         */

        /*
         * Order prior updates to reserve count prior to the
         * commit_cold cc_sb update.
         */
        cmm_smp_wmb();
        cc_cold = shmp_index(handle, buf->commit_cold, idx);
        if (!cc_cold)
                return;
        if (caa_likely(v_cmpxchg(config, &cc_cold->cc_sb,
                                 old_commit_count, old_commit_count + 1)
                   == old_commit_count)) {
                uint64_t *ts_end;

                /*
                 * Start of exclusive subbuffer access. We are
                 * guaranteed to be the last writer in this subbuffer
                 * and any other writer trying to access this subbuffer
                 * in this state is required to drop records.
                 *
                 * We can read the ts_end for the current sub-buffer
                 * which has been saved by the very last space
                 * reservation for the current sub-buffer.
                 *
                 * Order increment of commit counter before reading ts_end.
                 */
                cmm_smp_mb();
                ts_end = shmp_index(handle, buf->ts_end, idx);
                if (!ts_end)
                        return;
                deliver_count_events(config, buf, idx, handle);
                config->cb.buffer_end(buf, *ts_end, idx,
                                      lib_ring_buffer_get_data_size(config,
                                                                buf,
                                                                idx,
                                                                handle),
                                      handle);

                /*
                 * Increment the packet counter while we have exclusive
                 * access.
                 */
                subbuffer_inc_packet_count(config, &buf->backend, idx, handle);

                /*
                 * Set noref flag and offset for this subbuffer id.
                 * Contains a memory barrier that ensures counter stores
                 * are ordered before set noref and offset.
                 */
                lib_ring_buffer_set_noref_offset(config, &buf->backend, idx,
                                                 buf_trunc_val(offset, chan), handle);

                /*
                 * Order set_noref and record counter updates before the
                 * end of subbuffer exclusive access. Orders with
                 * respect to writers coming into the subbuffer after
                 * wrap around, and also order wrt concurrent readers.
                 */
                cmm_smp_mb();
                /* End of exclusive subbuffer access */
                v_set(config, &cc_cold->cc_sb, commit_count);
                /*
                 * Order later updates to reserve count after
                 * the commit cold cc_sb update.
                 */
                cmm_smp_wmb();
                lib_ring_buffer_vmcore_check_deliver(config, buf,
                                         commit_count, idx, handle);

                /*
                 * RING_BUFFER_WAKEUP_BY_WRITER wakeup is not lock-free.
                 */
                if (config->wakeup == RING_BUFFER_WAKEUP_BY_WRITER
                    && uatomic_read(&buf->active_readers)
                    && lib_ring_buffer_poll_deliver(config, buf, chan, handle)) {
                        lib_ring_buffer_wakeup(buf, handle);
                }
        }
}

/*
 * Force a read (imply TLS fixup for dlopen) of TLS variables.
 */
void lttng_fixup_ringbuffer_tls(void)
{
        asm volatile ("" : : "m" (URCU_TLS(lib_ring_buffer_nesting)));
}

void lib_ringbuffer_signal_init(void)
{
        sigset_t mask;
        int ret;

        /*
         * Block signal for entire process, so only our thread processes
         * it.
         */
        rb_setmask(&mask);
        ret = pthread_sigmask(SIG_BLOCK, &mask, NULL);
        if (ret) {
                errno = ret;
                PERROR("pthread_sigmask");
        }
}