Re: [PATCH 4/4] ALSA: pcm_dmaengine: Properly synchronize DMA on shutdown

[Takashi Iwai <tiwai@suse.de>]

On Tue, 20 Oct 2015 13:40:00 +0200,
Lars-Peter Clausen wrote:
> 
> On 10/20/2015 01:17 PM, Takashi Iwai wrote:
> > On Tue, 20 Oct 2015 11:46:31 +0200,
> > Lars-Peter Clausen wrote:
> >>
> >> Use the new dmaengine_synchronize() function to make sure that all complete
> >> callbacks have finished running before the runtime data, which is accessed
> >> in the completed callback, is freed.
> >>
> >> This fixes a long standing use-after-free race condition that has been
> >> observed on some systems.
> > 
> > What if a substream is restarted immediately after the stop?
> > 
> 
> What can happen is that you get a complete callback and the associated
> snd_pcm_period_elapsed() too early, before the period has actually elapsed,
> but I don't think that this is a problem if the DMA driver properly
> implements residue reporting.
> 
> This fails if we rely on period counting, but that is broken anyway and
> already prone to other race conditions.
> 
> I've tested this series with xrun injection and some modifications to the
> DMA driver to always trigger the race condition when the stream is stopped.
> And I've not seen any issues after the transfer re-started. (There is a
> dead-lock condition though but that does not seem to be related to this series)

OK, then I'm fine with the changes.

I suppose this will go through dmaengine tree?  If so, feel free to
take my ack:
  Reviewed-by: Takashi Iwai <tiwai@suse.de>


thanks,

Takashi