[alsa-devel] another locking issue with nonatomic ops?
Vinod Koul
vinod.koul at intel.com
Tue Feb 16 17:40:08 CET 2016
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.
> ---
> 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
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