[lttng-dev] [PATCH lttng-tools] Fix: UST subbuffers silently dropped on moderate trace traffic

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Tue Nov 18 11:33:23 EST 2014

Well, it looks like we really screwed up on this one.

lttng-tools commit 02b3d1769d5f8a33e4109b1e681141c9295dfda6 introduced
an important regression for lttng-ust tracing in the consumer daemon:
after reading a sub-buffer, a check has been added to see whether there
are more sub-buffers available to read, and if it is the case, it
ensures the wakeup pipe will be awakened again.

The issue lies in the use of ustctl_put_next_subbuf() in this check.
This acts as if the sub-buffer has been read, when in reality it has not
been read. It therefore trashes the data contained by this sub-buffer.

This check should use ustctl_put_subbuf(), which does not move the
consumer position.

This is a severe bug, and the fix needs to be applied to stable-2.6,
stable-2.5, and stable-2.4.

Fixes #861

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
 src/common/ust-consumer/ust-consumer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/common/ust-consumer/ust-consumer.c b/src/common/ust-consumer/ust-consumer.c
index 52b6979..24ba387 100644
--- a/src/common/ust-consumer/ust-consumer.c
+++ b/src/common/ust-consumer/ust-consumer.c
@@ -1882,7 +1882,7 @@ static int notify_if_more_data(struct lttng_consumer_stream *stream,
 		goto end;
-	ret = ustctl_put_next_subbuf(ustream);
+	ret = ustctl_put_subbuf(ustream);
 	/* This stream still has data. Flag it and wake up the data thread. */

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