X-Git-Url: http://git.liburcu.org/?a=blobdiff_plain;f=libringbuffer%2Fring_buffer_frontend.c;h=e631ae3dc36340c1af5427c75551f68310602379;hb=3d3c3833d4d119acde0a3fdb2c472aea010cd7e7;hp=ed60d7e22aa63ee127f2ae8b00f186522bdaae13;hpb=a33eb425f64228f7e2e545cb2a9d8840720657ae;p=lttng-ust.git

diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c
index ed60d7e2..e631ae3d 100644
--- a/libringbuffer/ring_buffer_frontend.c
+++ b/libringbuffer/ring_buffer_frontend.c
@@ -52,6 +52,7 @@
  */
 
 #define _GNU_SOURCE
+#define _LGPL_SOURCE
 #include <sys/types.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
@@ -421,6 +422,113 @@ void lib_ring_buffer_channel_switch_timer(int sig, siginfo_t *si, void *uc)
 	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 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
+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 channel *chan)
 {
@@ -1488,7 +1596,8 @@ void lib_ring_buffer_print_errors(struct channel *chan,
 /*
  * lib_ring_buffer_switch_old_start: Populate old subbuffer header.
  *
- * Only executed when the buffer is finalized, in SWITCH_FLUSH.
+ * 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,
@@ -1500,6 +1609,7 @@ void lib_ring_buffer_switch_old_start(struct lttng_ust_lib_ring_buffer *buf,
 	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);
 
@@ -1508,15 +1618,16 @@ void lib_ring_buffer_switch_old_start(struct lttng_ust_lib_ring_buffer *buf,
 	 * determine that the subbuffer is full.
 	 */
 	cmm_smp_wmb();
+	cc_hot = shmp_index(handle, buf->commit_hot, oldidx);
 	v_add(config, config->cb.subbuffer_header_size(),
-	      &shmp_index(handle, buf->commit_hot, oldidx)->cc);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, oldidx)->cc);
+	      &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, oldidx,
+	lib_ring_buffer_write_commit_counter(config, buf, chan,
 			offsets->old + config->cb.subbuffer_header_size(),
-			commit_count, handle);
+			commit_count, handle, cc_hot);
 }
 
 /*
@@ -1537,6 +1648,7 @@ void lib_ring_buffer_switch_old_end(struct lttng_ust_lib_ring_buffer *buf,
 	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;
 
 	data_size = subbuf_offset(offsets->old - 1, chan) + 1;
 	padding_size = chan->backend.subbuf_size - data_size;
@@ -1548,12 +1660,14 @@ void lib_ring_buffer_switch_old_end(struct lttng_ust_lib_ring_buffer *buf,
 	 * determine that the subbuffer is full.
 	 */
 	cmm_smp_wmb();
-	v_add(config, padding_size, &shmp_index(handle, buf->commit_hot, oldidx)->cc);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, oldidx)->cc);
+	cc_hot = shmp_index(handle, buf->commit_hot, oldidx);
+	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, oldidx,
-			offsets->old + padding_size, commit_count, handle);
+	lib_ring_buffer_write_commit_counter(config, buf, chan,
+			offsets->old + padding_size, commit_count, handle,
+			cc_hot);
 }
 
 /*
@@ -1573,6 +1687,7 @@ void lib_ring_buffer_switch_new_start(struct lttng_ust_lib_ring_buffer *buf,
 	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);
 
@@ -1583,13 +1698,14 @@ void lib_ring_buffer_switch_new_start(struct lttng_ust_lib_ring_buffer *buf,
 	cmm_smp_wmb();
 	v_add(config, config->cb.subbuffer_header_size(),
 	      &shmp_index(handle, buf->commit_hot, beginidx)->cc);
-	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, beginidx)->cc);
+	cc_hot = shmp_index(handle, buf->commit_hot, beginidx);
+	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, beginidx,
+	lib_ring_buffer_write_commit_counter(config, buf, chan,
 			offsets->begin + config->cb.subbuffer_header_size(),
-			commit_count, handle);
+			commit_count, handle, cc_hot);
 }
 
 /*
@@ -1662,12 +1778,14 @@ int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
 		unsigned long sb_index, commit_count;
 
 		/*
-		 * We are performing a SWITCH_FLUSH. At this stage, there are no
-		 * concurrent writes into the buffer.
+		 * 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.
 		 *
-		 * The client does not save any header information.  Don't
-		 * switch empty subbuffer on finalize, because it is invalid to
-		 * deliver a completely empty subbuffer.
+		 * 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;
@@ -2046,6 +2164,149 @@ int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx)
 	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)
+{
+	if (config->oops == RING_BUFFER_OOPS_CONSISTENCY)
+		v_set(config, &shmp_index(handle, buf->commit_hot, idx)->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 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;
+
+	/*
+	 * 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();
+	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.
+		 */
+		deliver_count_events(config, buf, idx, handle);
+		config->cb.buffer_end(buf, tsc, 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, &shmp_index(handle, buf->commit_cold, idx)->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.
  */