X-Git-Url: http://git.lttng.org/?a=blobdiff_plain;f=libringbuffer%2Ffrontend_internal.h;h=f6d91bde2bf63f0917b9bd27b90e73ed54429bcc;hb=fb31eb73d8a4a6d9784ed5c335b7fa3b9684108c;hp=4ad921ec0ec8263086cddbf1c11044110dc672f6;hpb=beca55a1aa4ff272188d7c37c19eb0540f811c8b;p=lttng-ust.git

diff --git a/libringbuffer/frontend_internal.h b/libringbuffer/frontend_internal.h
index 4ad921ec..f6d91bde 100644
--- a/libringbuffer/frontend_internal.h
+++ b/libringbuffer/frontend_internal.h
@@ -34,6 +34,7 @@
 #include <urcu/compiler.h>
 #include <urcu/tls-compat.h>
 #include <signal.h>
+#include <stdint.h>
 #include <pthread.h>
 
 #include <lttng/ringbuffer-config.h>
@@ -205,6 +206,15 @@ void lib_ring_buffer_reserve_push_reader(struct lttng_ust_lib_ring_buffer *buf,
 					      consumed_new) != consumed_old));
 }
 
+/*
+ * Move consumed position to the beginning of subbuffer in which the
+ * write offset is. Should only be used on ring buffers that are not
+ * actively being written into, because clear_reader does not take into
+ * account the commit counters when moving the consumed position, which
+ * can make concurrent trace producers or consumers observe consumed
+ * position further than the write offset, which breaks ring buffer
+ * algorithm guarantees.
+ */
 static inline
 void lib_ring_buffer_clear_reader(struct lttng_ust_lib_ring_buffer *buf,
 				  struct lttng_ust_shm_handle *handle)
@@ -221,12 +231,10 @@ void lib_ring_buffer_clear_reader(struct lttng_ust_lib_ring_buffer *buf,
 	do {
 		offset = v_read(config, &buf->offset);
 		consumed_old = uatomic_read(&buf->consumed);
-		if (caa_unlikely(subbuf_trunc(offset, chan)
-			      - subbuf_trunc(consumed_old, chan)
-			     > 0))
-			consumed_new = subbuf_trunc(offset, chan);
-		else
-			return;
+		CHAN_WARN_ON(chan, (long) (subbuf_trunc(offset, chan)
+				- subbuf_trunc(consumed_old, chan))
+				< 0);
+		consumed_new = subbuf_trunc(offset, chan);
 	} while (caa_unlikely(uatomic_cmpxchg(&buf->consumed, consumed_old,
 					      consumed_new) != consumed_old));
 }
@@ -260,14 +268,11 @@ int lib_ring_buffer_reserve_committed(const struct lttng_ust_lib_ring_buffer_con
 				      struct lttng_ust_shm_handle *handle)
 {
 	unsigned long offset, idx, commit_count;
-	struct commit_counters_hot *cc_hot = shmp_index(handle, buf->commit_hot, idx);
+	struct commit_counters_hot *cc_hot;
 
 	CHAN_WARN_ON(chan, config->alloc != RING_BUFFER_ALLOC_PER_CPU);
 	CHAN_WARN_ON(chan, config->sync != RING_BUFFER_SYNC_PER_CPU);
 
-	if (caa_unlikely(!cc_hot))
-		return 0;
-
 	/*
 	 * Read offset and commit count in a loop so they are both read
 	 * atomically wrt interrupts. By deal with interrupt concurrency by
@@ -279,6 +284,9 @@ int lib_ring_buffer_reserve_committed(const struct lttng_ust_lib_ring_buffer_con
 	do {
 		offset = v_read(config, &buf->offset);
 		idx = subbuf_index(offset, chan);
+		cc_hot = shmp_index(handle, buf->commit_hot, idx);
+		if (caa_unlikely(!cc_hot))
+			return 0;
 		commit_count = v_read(config, &cc_hot->cc);
 	} while (offset != v_read(config, &buf->offset));