[lttng-ust.git] / libringbuffer / frontend_internal.h

#ifndef _LTTNG_RING_BUFFER_FRONTEND_INTERNAL_H
#define _LTTNG_RING_BUFFER_FRONTEND_INTERNAL_H

/*
 * libringbuffer/frontend_internal.h
 *
 * Ring Buffer Library Synchronization Header (internal helpers).
 *
 * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; only
 * version 2.1 of the License.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 *
 *
 * Author:
 *      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * See ring_buffer_frontend.c for more information on wait-free algorithms.
 *
 * Dual LGPL v2.1/GPL v2 license.
 */

#include <urcu/compiler.h>
#include <urcu/tls-compat.h>
#include <signal.h>
#include <pthread.h>

#include <lttng/ringbuffer-config.h>
#include "backend_types.h"
#include "frontend_types.h"
#include "shm.h"

/* Buffer offset macros */

/* buf_trunc mask selects only the buffer number. */
static inline
unsigned long buf_trunc(unsigned long offset, struct channel *chan)
{
        return offset & ~(chan->backend.buf_size - 1);

}

/* Select the buffer number value (counter). */
static inline
unsigned long buf_trunc_val(unsigned long offset, struct channel *chan)
{
        return buf_trunc(offset, chan) >> chan->backend.buf_size_order;
}

/* buf_offset mask selects only the offset within the current buffer. */
static inline
unsigned long buf_offset(unsigned long offset, struct channel *chan)
{
        return offset & (chan->backend.buf_size - 1);
}

/* subbuf_offset mask selects the offset within the current subbuffer. */
static inline
unsigned long subbuf_offset(unsigned long offset, struct channel *chan)
{
        return offset & (chan->backend.subbuf_size - 1);
}

/* subbuf_trunc mask selects the subbuffer number. */
static inline
unsigned long subbuf_trunc(unsigned long offset, struct channel *chan)
{
        return offset & ~(chan->backend.subbuf_size - 1);
}

/* subbuf_align aligns the offset to the next subbuffer. */
static inline
unsigned long subbuf_align(unsigned long offset, struct channel *chan)
{
        return (offset + chan->backend.subbuf_size)
               & ~(chan->backend.subbuf_size - 1);
}

/* subbuf_index returns the index of the current subbuffer within the buffer. */
static inline
unsigned long subbuf_index(unsigned long offset, struct channel *chan)
{
        return buf_offset(offset, chan) >> chan->backend.subbuf_size_order;
}

/*
 * Last TSC comparison functions. Check if the current TSC overflows tsc_bits
 * bits from the last TSC read. When overflows are detected, the full 64-bit
 * timestamp counter should be written in the record header. Reads and writes
 * last_tsc atomically.
 */

#if (CAA_BITS_PER_LONG == 32)
static inline
void save_last_tsc(const struct lttng_ust_lib_ring_buffer_config *config,
                   struct lttng_ust_lib_ring_buffer *buf, uint64_t tsc)
{
        if (config->tsc_bits == 0 || config->tsc_bits == 64)
                return;

        /*
         * Ensure the compiler performs this update in a single instruction.
         */
        v_set(config, &buf->last_tsc, (unsigned long)(tsc >> config->tsc_bits));
}

static inline
int last_tsc_overflow(const struct lttng_ust_lib_ring_buffer_config *config,
                      struct lttng_ust_lib_ring_buffer *buf, uint64_t tsc)
{
        unsigned long tsc_shifted;

        if (config->tsc_bits == 0 || config->tsc_bits == 64)
                return 0;

        tsc_shifted = (unsigned long)(tsc >> config->tsc_bits);
        if (caa_unlikely(tsc_shifted
                     - (unsigned long)v_read(config, &buf->last_tsc)))
                return 1;
        else
                return 0;
}
#else
static inline
void save_last_tsc(const struct lttng_ust_lib_ring_buffer_config *config,
                   struct lttng_ust_lib_ring_buffer *buf, uint64_t tsc)
{
        if (config->tsc_bits == 0 || config->tsc_bits == 64)
                return;

        v_set(config, &buf->last_tsc, (unsigned long)tsc);
}

static inline
int last_tsc_overflow(const struct lttng_ust_lib_ring_buffer_config *config,
                      struct lttng_ust_lib_ring_buffer *buf, uint64_t tsc)
{
        if (config->tsc_bits == 0 || config->tsc_bits == 64)
                return 0;

        if (caa_unlikely((tsc - v_read(config, &buf->last_tsc))
                     >> config->tsc_bits))
                return 1;
        else
                return 0;
}
#endif

extern
int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx);

extern
void lib_ring_buffer_switch_slow(struct lttng_ust_lib_ring_buffer *buf,
                                 enum switch_mode mode,
                                 struct lttng_ust_shm_handle *handle);

/* Buffer write helpers */

static inline
void lib_ring_buffer_reserve_push_reader(struct lttng_ust_lib_ring_buffer *buf,
                                         struct channel *chan,
                                         unsigned long offset)
{
        unsigned long consumed_old, consumed_new;

        do {
                consumed_old = uatomic_read(&buf->consumed);
                /*
                 * If buffer is in overwrite mode, push the reader consumed
                 * count if the write position has reached it and we are not
                 * at the first iteration (don't push the reader farther than
                 * the writer). This operation can be done concurrently by many
                 * writers in the same buffer, the writer being at the farthest
                 * write position sub-buffer index in the buffer being the one
                 * which will win this loop.
                 */
                if (caa_unlikely(subbuf_trunc(offset, chan)
                              - subbuf_trunc(consumed_old, chan)
                             >= chan->backend.buf_size))
                        consumed_new = subbuf_align(consumed_old, chan);
                else
                        return;
        } while (caa_unlikely(uatomic_cmpxchg(&buf->consumed, consumed_old,
                                              consumed_new) != consumed_old));
}

static inline
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)
{
        if (config->oops == RING_BUFFER_OOPS_CONSISTENCY)
                v_set(config, &shmp_index(handle, buf->commit_hot, idx)->seq, commit_count);
}

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

        consumed_old = uatomic_read(&buf->consumed);
        consumed_idx = subbuf_index(consumed_old, chan);
        commit_count = v_read(config, &shmp_index(handle, buf->commit_cold, consumed_idx)->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 inline
int lib_ring_buffer_pending_data(const struct lttng_ust_lib_ring_buffer_config *config,
                                 struct lttng_ust_lib_ring_buffer *buf,
                                 struct channel *chan)
{
        return !!subbuf_offset(v_read(config, &buf->offset), chan);
}

static inline
unsigned long lib_ring_buffer_get_data_size(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)
{
        return subbuffer_get_data_size(config, &buf->backend, idx, handle);
}

/*
 * Check if all space reservation in a buffer have been committed. This helps
 * knowing if an execution context is nested (for per-cpu buffers only).
 * This is a very specific ftrace use-case, so we keep this as "internal" API.
 */
static inline
int lib_ring_buffer_reserve_committed(const struct lttng_ust_lib_ring_buffer_config *config,
                                      struct lttng_ust_lib_ring_buffer *buf,
                                      struct channel *chan,
                                      struct lttng_ust_shm_handle *handle)
{
        unsigned long offset, idx, commit_count;

        CHAN_WARN_ON(chan, config->alloc != RING_BUFFER_ALLOC_PER_CPU);
        CHAN_WARN_ON(chan, config->sync != RING_BUFFER_SYNC_PER_CPU);

        /*
         * Read offset and commit count in a loop so they are both read
         * atomically wrt interrupts. By deal with interrupt concurrency by
         * restarting both reads if the offset has been pushed. Note that given
         * we only have to deal with interrupt concurrency here, an interrupt
         * modifying the commit count will also modify "offset", so it is safe
         * to only check for offset modifications.
         */
        do {
                offset = v_read(config, &buf->offset);
                idx = subbuf_index(offset, chan);
                commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, idx)->cc);
        } while (offset != v_read(config, &buf->offset));

        return ((buf_trunc(offset, chan) >> chan->backend.num_subbuf_order)
                     - (commit_count & chan->commit_count_mask) == 0);
}

static inline
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 inline
void lib_ring_buffer_check_deliver(const struct lttng_ust_lib_ring_buffer_config *config,
                                   struct lttng_ust_lib_ring_buffer *buf,
                                   struct channel *chan,
                                   unsigned long offset,
                                   unsigned long commit_count,
                                   unsigned long idx,
                                   struct lttng_ust_shm_handle *handle)
{
        unsigned long old_commit_count = commit_count
                                         - chan->backend.subbuf_size;
        uint64_t tsc;

        /* Check if all commits have been done */
        if (caa_unlikely((buf_trunc(offset, chan) >> chan->backend.num_subbuf_order)
                     - (old_commit_count & chan->commit_count_mask) == 0)) {
                /*
                 * 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.
                 */
                if (caa_likely(v_cmpxchg(config, &shmp_index(handle, buf->commit_cold, idx)->cc_sb,
                                         old_commit_count, old_commit_count + 1)
                           == old_commit_count)) {
                        /*
                         * 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.
                         */
                        tsc = config->cb.ring_buffer_clock_read(chan);
                        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);
                        config->cb.buffer_end(buf, tsc, idx,
                                              lib_ring_buffer_get_data_size(config,
                                                                        buf,
                                                                        idx,
                                                                        handle),
                                              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, &shmp_index(handle, buf->commit_cold, idx)->cc_sb,
                              commit_count);
                        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);
                        }
                }
        }
}

/*
 * lib_ring_buffer_write_commit_counter
 *
 * For flight recording. must be called after commit.
 * This function increments the subbuffer's commit_seq counter each time the
 * commit count reaches back the reserve offset (modulo subbuffer size). It is
 * useful for crash dump.
 */
static inline
void lib_ring_buffer_write_commit_counter(const struct lttng_ust_lib_ring_buffer_config *config,
                                          struct lttng_ust_lib_ring_buffer *buf,
                                          struct channel *chan,
                                          unsigned long idx,
                                          unsigned long buf_offset,
                                          unsigned long commit_count,
                                          size_t slot_size,
                                          struct lttng_ust_shm_handle *handle)
{
        unsigned long offset, commit_seq_old;

        if (config->oops != RING_BUFFER_OOPS_CONSISTENCY)
                return;

        offset = buf_offset + slot_size;

        /*
         * subbuf_offset includes commit_count_mask. We can simply
         * compare the offsets within the subbuffer without caring about
         * buffer full/empty mismatch because offset is never zero here
         * (subbuffer header and record headers have non-zero length).
         */
        if (caa_unlikely(subbuf_offset(offset - commit_count, chan)))
                return;

        commit_seq_old = v_read(config, &shmp_index(handle, buf->commit_hot, idx)->seq);
        while ((long) (commit_seq_old - commit_count) < 0)
                commit_seq_old = v_cmpxchg(config, &shmp_index(handle, buf->commit_hot, idx)->seq,
                                           commit_seq_old, commit_count);
}

extern 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);
extern void lib_ring_buffer_free(struct lttng_ust_lib_ring_buffer *buf,
                                 struct lttng_ust_shm_handle *handle);

/* Keep track of trap nesting inside ring buffer code */
extern DECLARE_URCU_TLS(unsigned int, lib_ring_buffer_nesting);

#endif /* _LTTNG_RING_BUFFER_FRONTEND_INTERNAL_H */