[alsa-devel] another locking issue with nonatomic ops?

[Babu, Ramesh]

> On Tue, Feb 16, 2016 at 05:27:44PM +0100, Takashi Iwai wrote:
> > On Tue, 16 Feb 2016 17:10:43 +0100,
> > Vinod Koul wrote:
> > >
> > > On Tue, Feb 16, 2016 at 05:01:36PM +0100, Takashi Iwai wrote:
> > > > On Tue, 16 Feb 2016 16:37:11 +0100,
> > > > Vinod Koul wrote:
> > > > >
> > > > >
> > > > > Hi Takashi,
> > > > >
> > > > > We found another issue with non atomic ops.
> > > > >
> > > > > This occurs when we have snd_pcm_do_prepare being invoked
> which acquires
> > > > > the lock snd_pcm_link_rwsem twice, first in
> snd_pcm_common_ioctl1, and then again in
> > > > > dpcm_dapm_stream_event. Normally this is fine, but if we have
> another stream
> > > > > get closed in between two read accesses we get stuck.
> > > > >
> > > > > First thread:
> > > > >
> > > > > [  419.657259] AudioOut_2      D ffff8800704a3a60     0  3691      1
> 	0x20020004
> > > > > [  419.665946]  ffff8800704a3a60 00000000704a3a60 ffff88006d1fddd0
> ffffffff8252ffd8
> > > > > [  419.674678]  ffff8800704a4000 ffffffff8252ffc0 ffffffffffffffff
> ffffffff8252ffd8
> > > > > [  419.683513]  0000000000000000 ffff8800704a3a80 ffffffff81ca2987
> ffffffffffffffff
> > > > > [  419.683574] Call Trace:
> > > > > [  419.692290]  [<ffffffff81ca2987>] schedule+0x37/0x90
> > > > > [  419.692306]  [<ffffffff81ca6f6d>]
> rwsem_down_read_failed+0xdd/0x130
> > > > > [  419.701050]  [<ffffffff814df454>]
> call_rwsem_down_read_failed+0x14/0x30
> > > > > [  419.709758]  [<ffffffff810e793b>] ? down_read_nested+0x5b/0x70
> > > > > [  419.709805]  [<ffffffff81adfda9>] ?
> snd_pcm_stream_lock+0xa9/0x120
> > > > > [  419.723012]  [<ffffffff81adfda9>]
> snd_pcm_stream_lock+0xa9/0x120
> > > > >
> > > > > < ============     2nd Read lock is acquired here, without releasing
> previous one.
> > > > >
> > > > > [  419.723026]  [<ffffffff81adfe4c>]
> snd_pcm_stream_lock_irq+0x2c/0x30
> > > > > [  419.731801]  [<ffffffff81b01f5e>] ?
> dpcm_dapm_stream_event+0xbe/0xd0
> > > > > [  419.740565]  [<ffffffff81b0079c>]
> dpcm_set_fe_update_state+0x3c/0xb0
> > > > > [  419.749252]  [<ffffffff81b04514>]
> dpcm_fe_dai_prepare+0xc4/0x150
> > > > > [  419.749301]  [<ffffffff81adf7eb>] snd_pcm_do_prepare+0x1b/0x30
> > > > > [  419.758083]  [<ffffffff81adf3bf>] snd_pcm_action_single+0x2f/0x70
> > > > > [  419.766897]  [<ffffffff81ca6827>] ? down_read+0x47/0x60
> > > > > [  419.766962]  [<ffffffff81adf577>] ?
> snd_pcm_action_nonatomic+0x27/0x80
> > > > > [  419.775565]  [<ffffffff81adf5c6>]
> snd_pcm_action_nonatomic+0x76/0x80
> > > > >
> > > > > < ============     1st Read lock is acquired here
> > > > >
> > > > > [  419.784419]  [<ffffffff81ae2c62>]
> snd_pcm_common_ioctl1+0x802/0xd30
> > > > > [  419.784495]  [<ffffffff81ae332b>]
> snd_pcm_playback_ioctl1+0x19b/0x280
> > > > > [  419.793106]  [<ffffffff81230915>] ? __fget+0x5/0x210
> > > > > [  419.801943]  [<ffffffff81ae3dca>]
> snd_pcm_ioctl_compat+0x41a/0x770
> > > > > [  419.801959]  [<ffffffff81230915>] ? __fget+0x5/0x210
> > > > > [  419.810793]  [<ffffffff8126d670>] compat_SyS_ioctl+0xd0/0x13b0
> > > > > [  419.819491]  [<ffffffff81ca7bc4>] ? do_nanosleep+0x94/0x190
> > > > > [  419.819535]  [<ffffffff810eb696>] ?
> trace_hardirqs_on_caller+0x16/0x210
> > > > > [  419.828306]  [<ffffffff814df4f8>] ?
> trace_hardirqs_on_thunk+0x17/0x19
> > > > > [  419.837104]  [<ffffffff81cabf0e>] sysenter_dispatch+0x7/0x1f
> > > > >
> > > > >
> > > > > Second thread:
> > > > > [  419.543062] mediaserver     D ffff880170957cd8     0  3690      1
> 	0x20020004
> > > > > [  419.543191]  ffff880170957cd8 0000000070957cd8 ffff88006de49ad0
> ffffffff8252ffd8
> > > > > [  419.551920]  ffff880170958000 ffff88006de49ad0 ffffffff8252ffc0
> ffffffff00000001
> > > > > [  419.560555]  ffffffff8252ffd8 ffff880170957cf8 ffffffff81ca2987
> ffffffff8252ffc0
> > > > > [  419.569368] Call Trace:
> > > > > [  419.569392]  [<ffffffff81ca2987>] schedule+0x37/0x90
> > > > > [  419.578026]  [<ffffffff81ca71f5>]
> rwsem_down_write_failed+0x235/0x450
> > > > > [  419.586884]  [<ffffffff81ca7005>] ?
> rwsem_down_write_failed+0x45/0x450
> > > > > [  419.586941]  [<ffffffff814df483>]
> call_rwsem_down_write_failed+0x13/0x20
> > > > > [  419.595620]  [<ffffffff81ca689f>] ? down_write+0x5f/0x80
> > > > >
> > > > > < ============     Write lock acquired here. If this occurs between
> two read
> > > > > lock's then it's hangs
> > > >
> > > > So what's wrong?  The two read locks are still running, and
> > > > snd_pcm_release() waits until these two read locks are freed.
> > > > The read double-locks are fine, and it's intended behavior.
> > >
> > > Yes that is true
> > >
> > > > The problem is apparently not there, but somewhere else.
> > > > Any other thread or call involved?
> > >
> > > The second backtrace above is the 2nd thread which does down_write.
> That
> > > gets hung as 1st read lock is acquired. Since write gets higer priorty, 2nd
> > > read is also stuck and we are in deadlock.
> > >
> > > down_read (1st acquired)
> > > 					down_write (waiting due to
> down_read)
> > > down_read( 2nd,,, stuck)
> >
> > Hmm, so down_read() takes care of the pending down_write()?
> > Then it's a problem.  Maybe the limitation of the current rwsem
> > implementation.
> 
> Yes that is how things seem right now.
> >
> > Does the patch below work?  It's suboptimal, just a proof of concept.
> 
> Okay will try first thing tomorrow and let you know.
> 
I tried the below changes and it seem to be fixing the issue.
I will do more stress testing tomorrow and let you know.
Thanks :)

> > ---
> > diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> > index fadd3eb8e8bb..8b45edd218a4 100644
> > --- a/sound/core/pcm_native.c
> > +++ b/sound/core/pcm_native.c
> > @@ -1813,7 +1813,8 @@ static int snd_pcm_link(struct
> snd_pcm_substream *substream, int fd)
> >  		res = -ENOMEM;
> >  		goto _nolock;
> >  	}
> > -	down_write(&snd_pcm_link_rwsem);
> > +	while (!down_write_trylock(&snd_pcm_link_rwsem))
> > +		cpu_relax();
> >  	write_lock_irq(&snd_pcm_link_rwlock);
> >  	if (substream->runtime->status->state ==
> SNDRV_PCM_STATE_OPEN ||
> >  	    substream->runtime->status->state != substream1->runtime-
> >status->state ||
> > @@ -1860,7 +1861,8 @@ static int snd_pcm_unlink(struct
> snd_pcm_substream *substream)
> >  	struct snd_pcm_substream *s;
> >  	int res = 0;
> >
> > -	down_write(&snd_pcm_link_rwsem);
> > +	while (!down_write_trylock(&snd_pcm_link_rwsem))
> > +		cpu_relax();
> >  	write_lock_irq(&snd_pcm_link_rwlock);
> >  	if (!snd_pcm_stream_linked(substream)) {
> >  		res = -EALREADY;
> 
> --
> ~Vinod