linux-kernel - Re: sound: uninterruptible hang in snd_seq_oss_writeq

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Date:	Tue, 01 Mar 2016 16:31:10 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	alsa-devel@...a-project.org, Jaroslav Kysela <perex@...ex.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: 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@...gle.com> wrote:
> > On Tue, Mar 1, 2016 at 3:44 PM, Takashi Iwai <tiwai@...e.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;
 }
 
 /*