From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alan Young <Alan.Young@IEE.org>
Subject: Locking/mutex for snd_pcm_update_hw_ptr0() call
Date: Mon, 7 Nov 2016 12:53:08 +0000
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As far as I can see, snd_pcm_update_hw_ptr0() is a critical section of 
code that is protected by some form of mutex in all its call paths:

     snd_pcm_period_elapsed() - snd_pcm_stream_lock_irqsave()

     snd_pcm_update_hw_ptr()

         snd_pcm_lib_ioctl_reset() - snd_pcm_stream_lock_irqsave()

         snd_pcm_lib_write1() - snd_pcm_stream_lock_irq()

         snd_pcm_lib_read1() - snd_pcm_stream_lock_irq()

         snd_pcm_status() - snd_pcm_stream_lock_irq()

         snd_pcm_do_pause - I think via the locking in snd_pcm_action()

         snd_pcm_playback_rewind() - snd_pcm_stream_lock_irq()

         snd_pcm_capture_rewind() - snd_pcm_stream_lock_irq()

         snd_pcm_playback_forward() - snd_pcm_stream_lock_irq()

         snd_pcm_capture_forward() - snd_pcm_stream_lock_irq()

         snd_pcm_hwsync() - snd_pcm_stream_lock_irq()

         snd_pcm_delay() - snd_pcm_stream_lock_irq()



However, I am not familiar with how kernel locking mechanisms work. It 
is possible that the DMA interrupt handler (snd_pcm_period_elapsed()), 
could call snd_pcm_update_hw_ptr0() while it is part way through a call 
from snd_pcm_status(), or vice versa?

Alan.