linux-kernel - Re: sound: deadlock involving snd_hrtimer

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Date:	Sun, 24 Apr 2016 18:16:12 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	syzkaller <syzkaller@...glegroups.com>
Subject: Re: sound: deadlock involving snd_hrtimer_callback

On Sun, 24 Apr 2016 17:16:32 +0200,
Dmitry Vyukov wrote:
> 
> On Sat, Apr 23, 2016 at 11:02 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > On Sat, 23 Apr 2016 15:40:21 +0200,
> > Dmitry Vyukov wrote:
> >>
> >> Hi Takashi,
> >>
> >> I've incorporated your hrtimer fixes (but also updated to
> >> ddce192106e4f984123884f8e878f66ace94b573) and now I am seeing lots of
> >> the following deadlock messages:
> >>
> >>
> >> [ INFO: possible circular locking dependency detected ]
> >> 4.6.0-rc4+ #351 Not tainted
> >> -------------------------------------------------------
> >> swapper/0/0 is trying to acquire lock:
> >>  (&(&timer->lock)->rlock){-.-...}, at: [<ffffffff8537a749>]
> >> snd_timer_interrupt+0xa9/0xd30 sound/core/timer.c:701
> >>
> >> but task is already holding lock:
> >>  (&(&stime->lock)->rlock){-.....}, at: [<ffffffff85383d3f>]
> >> snd_hrtimer_callback+0x4f/0x2b0 sound/core/hrtimer.c:54
> >>
> >> which lock already depends on the new lock.
> >
> > Oh crap, my second patch is buggy, it leads to ABBA lock, indeed.
> > The first patch is still OK, as it just adds a new behavior mode.
> >
> > Could you replace the second patch with the below one?
> 
> 
> I've replaced the second path with this one. The deadlocks has gone,
> but I've hit these two hangs that look related:
> 
> https://gist.githubusercontent.com/dvyukov/805718ea249c49d17ae759d1b0160684/raw/20891f7e87fe9af3967565559d465d296469244b/gistfile1.txt
> https://gist.githubusercontent.com/dvyukov/7f397ea4aeb9e35596e0c8053cf35a11/raw/3fc22f24f7bab5941e47bab604f96487b5f1944d/gistfile1.txt

Hmm, so it wasn't a good idea to call hrtimer_cancel() in the
spinlock, in anyway.  Scratch the previous one.

OK, below is the yet revised two patches.  One is the simplified
version of the patch, and another is to call hrtimer_cancel() in a new
timer op without spinlock.  Apply these after the first patch
"ALSA: timer: Allow backend disabling start/stop from handler".


thanks,

Takashi


Download attachment "0001-ALSA-hrtimer-Use-manual-start-stop-in-callback-v3.patch" of type "application/octet-stream" (1510 bytes)

Download attachment "0002-ALSA-timer-Introduce-stop_sync-op.patch" of type "application/octet-stream" (4824 bytes)