X-Git-Url: http://git.liburcu.org/?a=blobdiff_plain;f=lib%2Fringbuffer%2Ffrontend_internal.h;fp=lib%2Fringbuffer%2Ffrontend_internal.h;h=048386146c1177ee525792b0a4849f71fe87ebd9;hb=ac4a87e53febd0dfc438ce75948e57d649d3b113;hp=a1bfe558440c7d8f7f5af4d45395d6047fe334b7;hpb=b6903d57e4c3234ec5b1c7f72e232023cdee0fab;p=lttng-modules.git

diff --git a/lib/ringbuffer/frontend_internal.h b/lib/ringbuffer/frontend_internal.h
index a1bfe558..04838614 100644
--- a/lib/ringbuffer/frontend_internal.h
+++ b/lib/ringbuffer/frontend_internal.h
@@ -189,7 +189,12 @@ void lib_ring_buffer_reserve_push_reader(struct lib_ring_buffer *buf,
 
 /*
  * Move consumed position to the beginning of subbuffer in which the
- * write offset is.
+ * 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 lib_ring_buffer *buf,
@@ -201,12 +206,10 @@ void lib_ring_buffer_clear_reader(struct lib_ring_buffer *buf,
 	do {
 		offset = v_read(config, &buf->offset);
 		consumed_old = atomic_long_read(&buf->consumed);
-		if (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 (unlikely(atomic_long_cmpxchg(&buf->consumed, consumed_old,
 					      consumed_new) != consumed_old));
 }