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SubjectRe: sound: uninterruptible hang in snd_seq_oss_writeq_sync
On Tue, 01 Mar 2016 16:04:43 +0100,
Dmitry Vyukov wrote:
>
> On Tue, Mar 1, 2016 at 4:01 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> > On Tue, Mar 1, 2016 at 3:44 PM, Takashi Iwai <tiwai@suse.de> wrote:
> >> On Tue, 01 Mar 2016 13:33:27 +0100,
> >> Dmitry Vyukov wrote:
> >>>
> >>> Hello,
> >>>
> >>> The following program creates an unkillable process:
> >> .....
> >>> The hang stack is:
> >>>
> >>> [<ffffffff85309f77>] snd_seq_oss_writeq_sync+0x327/0x790
> >>> sound/core/seq/oss/seq_oss_writeq.c:121
> >>
> >> This is
> >> wait_event_interruptible_timeout(q->sync_sleep, ! q->sync_event_put, HZ);
> >>
> >> and this should return zero
> >> if (signal_pending(current))
> >> /* interrupted - return 0 to finish sync */
> >> q->sync_event_put = 0;
> >> if (! q->sync_event_put || q->sync_time >= time)
> >> return 0;
> >> return 1;
> >>
> >>> [<ffffffff852fa353>] snd_seq_oss_drain_write+0x113/0x160
> >>
> >> ... and this loop should break:
> >> while (snd_seq_oss_writeq_sync(dp->writeq))
> >> ;
> >>
> >> So, I see no obvious error in the code, so far.
> >>
> >> I'm running your test program now with 8 parallel runs, but I couldn't
> >> reproduce it. Any other specifics?
>
>
> Hummm.. for me this process hangs every time, even if I just run it
> once from console.
> I've now retested in on clean commit
> fc77dbd34c5c99bce46d40a2491937c3bcbd10af (without KASAN/KCOV and with
> release gcc) and also got the same hang.
> I can only think of a different config. I've attached mine, please try with it.
>
> Are signals delivered if we are already in process of dying?
> When I kill -9 this process it does not react, so presumably wait is
> not unblocked...

Good point.

But above all, it doesn't make much sense to loop this sync call.
When wait_event*() returns, it should go out. So, the patch like
below should be good enough and fix your issue, too.


Takashi

---
diff --git a/sound/core/seq/oss/seq_oss_writeq.c b/sound/core/seq/oss/seq_oss_writeq.c
index 1f6788a18444..4014a5469d1a 100644
--- a/sound/core/seq/oss/seq_oss_writeq.c
+++ b/sound/core/seq/oss/seq_oss_writeq.c
@@ -119,12 +119,7 @@ snd_seq_oss_writeq_sync(struct seq_oss_writeq *q)
}

wait_event_interruptible_timeout(q->sync_sleep, ! q->sync_event_put, HZ);
- if (signal_pending(current))
- /* interrupted - return 0 to finish sync */
- q->sync_event_put = 0;
- if (! q->sync_event_put || q->sync_time >= time)
- return 0;
- return 1;
+ return 0;
}

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