Date:	Tue, 01 Mar 2016 18:43:39 +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 16:45:22 +0100,
Dmitry Vyukov wrote:
> 
> On Tue, Mar 1, 2016 at 4:31 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > 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.
> 
> 
> 
> The patch fixes the hang for me. There is a second delay at exit, but
> otherwise the process exits fine.
> 
> Tested-by: Dmitry Vyukov <dvyukov@...gle.com>
> 
> Thanks for quick fix!

OK, I got the same result finally on my VM, too.  I had to disable
INPUT_PCSPKR that conflicts with SND_PCSP.

I reconsidered about this problem again, and concluded that the
currently implemented drain behavior is simply superfluous.  So, the
fix is just to remove all these relevant calls.  The patch is attached
below.  Try this one instead of the previous one.


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai@...e.de>
Subject: [PATCH] ALSA: seq: oss: Don't drain at closing a client

The OSS sequencer client tries to drain the pending events at
releasing.  Unfortunately, as spotted by syzkaller fuzzer, this may
lead to an unkillable process state when the event has been queued at
the far future.  Since the process being released can't be signaled
any longer, it remains and waits for the echo-back event in that far
future.

Back to history, the draining feature was implemented at the time we
misinterpreted POSIX definition for blocking file operation.
Actually, such a behavior is superfluous at release, and we should
just release the device as is instead of keeping it up forever.

This patch just removes the draining call that may block the release
for too long time unexpectedly.

BugLink: http://lkml.kernel.org/r/CACT4Y+Y4kD-aBGj37rf-xBw9bH3GMU6P+MYg4W1e-s-paVD2pg@mail.gmail.com
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
---
 sound/core/seq/oss/seq_oss.c        |  2 --
 sound/core/seq/oss/seq_oss_device.h |  1 -
 sound/core/seq/oss/seq_oss_init.c   | 16 ----------------
 3 files changed, 19 deletions(-)

diff --git a/sound/core/seq/oss/seq_oss.c b/sound/core/seq/oss/seq_oss.c
index 8db156b207f1..8cdf489df80e 100644
--- a/sound/core/seq/oss/seq_oss.c
+++ b/sound/core/seq/oss/seq_oss.c
@@ -149,8 +149,6 @@ odev_release(struct inode *inode, struct file *file)
 	if ((dp = file->private_data) == NULL)
 		return 0;
 
-	snd_seq_oss_drain_write(dp);
-
 	mutex_lock(&register_mutex);
 	snd_seq_oss_release(dp);
 	mutex_unlock(&register_mutex);
diff --git a/sound/core/seq/oss/seq_oss_device.h b/sound/core/seq/oss/seq_oss_device.h
index b43924325249..d7b4d016b547 100644
--- a/sound/core/seq/oss/seq_oss_device.h
+++ b/sound/core/seq/oss/seq_oss_device.h
@@ -127,7 +127,6 @@ int snd_seq_oss_write(struct seq_oss_devinfo *dp, const char __user *buf, int co
 unsigned int snd_seq_oss_poll(struct seq_oss_devinfo *dp, struct file *file, poll_table * wait);
 
 void snd_seq_oss_reset(struct seq_oss_devinfo *dp);
-void snd_seq_oss_drain_write(struct seq_oss_devinfo *dp);
 
 /* */
 void snd_seq_oss_process_queue(struct seq_oss_devinfo *dp, abstime_t time);
diff --git a/sound/core/seq/oss/seq_oss_init.c b/sound/core/seq/oss/seq_oss_init.c
index 6779e82b46dd..92c96a95a903 100644
--- a/sound/core/seq/oss/seq_oss_init.c
+++ b/sound/core/seq/oss/seq_oss_init.c
@@ -436,22 +436,6 @@ snd_seq_oss_release(struct seq_oss_devinfo *dp)
 
 
 /*
- * Wait until the queue is empty (if we don't have nonblock)
- */
-void
-snd_seq_oss_drain_write(struct seq_oss_devinfo *dp)
-{
-	if (! dp->timer->running)
-		return;
-	if (is_write_mode(dp->file_mode) && !is_nonblock_mode(dp->file_mode) &&
-	    dp->writeq) {
-		while (snd_seq_oss_writeq_sync(dp->writeq))
-			;
-	}
-}
-
-
-/*
  * reset sequencer devices
  */
 void
-- 
2.7.2

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