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

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Date:	Tue, 1 Mar 2016 16:04:43 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Takashi Iwai <tiwai@...e.de>
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, 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...

Download attachment ".config" of type "application/octet-stream" (138698 bytes)