linux-kernel - [PATCH 3.19 093/101] [media] vb2: fix UNBALANCED warnings when calling vb2_thread

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Date:	Fri, 17 Apr 2015 15:29:21 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Jurgen Kramer <gtmkramer@...all.nl>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>
Subject: [PATCH 3.19 093/101] [media] vb2: fix UNBALANCED warnings when calling vb2_thread_stop()

3.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Hans Verkuil <hverkuil@...all.nl>

commit 0e661006370b7e7fb9ac9d94f9c3500a62cd559b upstream.

Stopping the vb2 thread (as used by several DVB devices) can result
in an 'UNBALANCED' warning such as this:

vb2: counters for queue ffff880407ee9828: UNBALANCED!
vb2:     setup: 1 start_streaming: 1 stop_streaming: 1
vb2:     wait_prepare: 249333 wait_finish: 249334

This is due to a race condition between stopping the thread and
calling vb2_internal_streamoff(). While I have not been able to deduce
the exact mechanism how this race condition can produce this warning,
I can see that the way the stream is stopped is likely to lead to a
race somewhere.

This patch simplifies how this is done by first ensuring that the
thread is completely stopped before cleaning up the vb2 queue. It
does that by setting threadio->stop to true, followed by a call to
vb2_queue_error() which will wake up the thread. The thread sees that
'stop' is true and it will exit.

The call to kthread_stop() waits until the thread has exited, and only
then is the queue cleaned up by calling __vb2_cleanup_fileio().

This is a much cleaner sequence and the warning has now disappeared.

Reported-by: Jurgen Kramer <gtmkramer@...all.nl>
Tested-by: Jurgen Kramer <gtmkramer@...all.nl>
Signed-off-by: Hans Verkuil <hans.verkuil@...co.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@....samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/media/v4l2-core/videobuf2-core.c |   11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

--- a/drivers/media/v4l2-core/videobuf2-core.c
+++ b/drivers/media/v4l2-core/videobuf2-core.c
@@ -3230,18 +3230,13 @@ int vb2_thread_stop(struct vb2_queue *q)

 	if (threadio == NULL)
 		return 0;
-	call_void_qop(q, wait_finish, q);
 	threadio->stop = true;
-	vb2_internal_streamoff(q, q->type);
-	call_void_qop(q, wait_prepare, q);
+	/* Wake up all pending sleeps in the thread */
+	vb2_queue_error(q);
 	err = kthread_stop(threadio->thread);
-	q->fileio = NULL;
-	fileio->req.count = 0;
-	vb2_reqbufs(q, &fileio->req);
-	kfree(fileio);
+	__vb2_cleanup_fileio(q);
 	threadio->thread = NULL;
 	kfree(threadio);
-	q->fileio = NULL;
 	q->threadio = NULL;
 	return err;
 }

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