[RFC][PATCH] sound/virtio: Fix cancel_sync warnings on uninitialized work_structs

[John Stultz <jstultz@xxxxxxxxxx>]

Betty reported hitting the following warning:

[    8.709131][  T221] WARNING: CPU: 2 PID: 221 at kernel/workqueue.c:4182
...
[    8.713282][  T221] Call trace:
[    8.713365][  T221]  __flush_work+0x8d0/0x914
[    8.713468][  T221]  __cancel_work_sync+0xac/0xfc
[    8.713570][  T221]  cancel_work_sync+0x24/0x34
[    8.713667][  T221]  virtsnd_remove+0xa8/0xf8 [virtio_snd ab15f34d0dd772f6d11327e08a81d46dc9c36276]
[    8.713868][  T221]  virtsnd_probe+0x48c/0x664 [virtio_snd ab15f34d0dd772f6d11327e08a81d46dc9c36276]
[    8.714035][  T221]  virtio_dev_probe+0x28c/0x390
[    8.714139][  T221]  really_probe+0x1bc/0x4c8
...

It seems we're hitting the error path in virtsnd_probe(), which
triggers a virtsnd_remove() which iterates over the substreams
calling cancel_work_sync() on the elapsed_period work_struct.

Looking at the code, from earlier in:
virtsnd_probe()->virtsnd_build_devs()->virtsnd_pcm_parse_cfg()

We set snd->nsubstreams, allocate the snd->substreams, and if
we then hit an error on the info allocation or something in
virtsnd_ctl_query_info() fails, we will exit without having
initialized the elapsed_period work_struct.

When that error path unwinds we then call virtsnd_remove()
which as long as the substreams array is allocated, will iterate
through calling cancel_work_sync() on the uninitialized work
struct hitting this warning.

The simplest fix seems to just check the work_struct->func ptr
is valid before calling cancel_work_sync(), but I wanted to share
in case folks had ideas for a better solution.

I have not yet managed to reproduce the issue myself, so this
patch has had limited testing.

Feedback or thoughts would be appreciated!

Cc: Anton Yakovlev <anton.yakovlev@xxxxxxxxxxxxxxx>
Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Jaroslav Kysela <perex@xxxxxxxx>
Cc: Takashi Iwai <tiwai@xxxxxxxx>
Cc: virtualization@xxxxxxxxxxxxxxx
Cc: linux-sound@xxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxxxxxxx
Reported-by: Betty Zhou <bettyzhou@xxxxxxxxxx>
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
---
 sound/virtio/virtio_card.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/sound/virtio/virtio_card.c b/sound/virtio/virtio_card.c
index 965209e1d872..87dc49c9390a 100644
--- a/sound/virtio/virtio_card.c
+++ b/sound/virtio/virtio_card.c
@@ -365,7 +365,9 @@ static void virtsnd_remove(struct virtio_device *vdev)
 	for (i = 0; snd->substreams && i < snd->nsubstreams; ++i) {
 		struct virtio_pcm_substream *vss = &snd->substreams[i];
 
-		cancel_work_sync(&vss->elapsed_period);
+		/* check we initialized the work struct before cancelling*/
+		if (vss->elapsed_period.func)
+			cancel_work_sync(&vss->elapsed_period);
 		virtsnd_pcm_msg_free(vss);
 	}
 
-- 
2.48.0.rc2.279.g1de40edade-goog