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

[Takashi Iwai]

On Tue, 16 Feb 2016 18:15:12 +0100,
Babu, Ramesh wrote:
> 
> > On Tue, 16 Feb 2016 17:40:19 +0100,
> > Babu, Ramesh 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.
> > > >
> > > down_write() will make sure no further read locks are allowed, till it get
> > > executed.
> > > So here the 2nd down_read() will get blocked, since down_write has
> > higher
> > > priority.
> > 
> > Well, the requirement of down_write() is to block further reads *once
> > when* it takes the lock.  It doesn't necessarily mean to block reads
> > *before* taking the write lock.
> 
> Below excerpts from LDD [1] says:
> "Writers get priority; as soon as a writer tries to enter the critical section,
> no readers will be allowed in until all writers have completed their work."

In your case, writer doesn't enter the critical section yet.  It's
trying to enter but blocked.

> My understanding when writer *tries* to acquire lock, further read locks
> will be blocked.  I think in my back trace, 2nd down_read() didn't get honored,
> since the down_write() was attempting to enter critical section from different
> thread.

Actually, this is pretty much depending on the implementation.  A
proper RT-wise rwsem may handle it differently, but the Linux standard
rwsem just queues a la FIFO, thus the writer blocks the rest.


Takashi