[alsa-devel] Issue in alsa when dma complete race with pcm release

[Shengjiu Wang]

On Tue, Jul 07, 2015 at 03:32:02PM +0200, Lars-Peter Clausen wrote:
> On 07/07/2015 12:13 PM, Shengjiu Wang wrote:
> >On Mon, Jul 06, 2015 at 02:27:01PM +0200, Lars-Peter Clausen wrote:
> >>On 07/06/2015 11:01 AM, Shengjiu Wang wrote:
> >>>On Fri, Jul 03, 2015 at 12:56:53PM +0200, Lars-Peter Clausen wrote:
> >>>>On 07/03/2015 10:25 AM, Shengjiu Wang wrote:
> >>>>>Hi alsa-devel
> >>>>>
> >>>>>    There maybe a issue in ALSA when dma complete race with snd_pcm_release.
> >>>>>The pcm release and dma complete are in different thread. There is occasion
> >>>>>that dmaengine_pcm_dma_complete() is called too late, some memory has been
> >>>>>freed, the prtd is null. Then there is kernel dump.
> >>>>>
> >>>>>    Is there any solution for this issue? Thanks.
> >>>>
> >>>>We need to introduce a synchronization primitive that allows a
> >>>>dmaengine client to synchronize to the execution of the complete
> >>>>callbacks.
> >>>>
> >>>>terminate_all() unfortunately can't do this since terminate_all()
> >>>>might be called from within one of the complete callbacks and so
> >>>>would cause a deadlock if we'd wait for all complete callbacks to
> >>>>finish before terminate_all() returns.
> >>>>
> >>>>So what is needed is a new function called dmaengine_sync() that
> >>>>will wait until all scheduled complete callbacks have finished. A
> >>>>call to this function needs to be put in snd_dmaengine_pcm_close()
> >>>>before the prtd is closed.
> >>>>
> >>>>- Lars
> >>>
> >>>How to check " all scheduled complete callbacks have finished"?
> >>
> >>That will be up to the dmaengine driver. But it basically comes down
> >>to two things:
> >>
> >>1) The driver needs to make sure that tasklet_schedule() is no
> >>longer called after terminate_all() has finished.
> >Some driver can't make sure this. The dma interrupt may come later after
> >terminate_all.
> 
> Most drivers can't make sure that the interrupt routine is not
> executed after terminate_all() has been called, since both are fully
> asynchronous. But what the driver needs to take care of is to
> synchronize the two e.g. using a spin_lock() and make sure that if
> terminate_all() has been called tasklet_schedule() is not called,
> even if the isr is executed. Most drivers get this rigght.
> 
> >>2) In the sync() callback call tasklet_kill() to make sure that it
> >>has finished running
> >>
> >>>
> >>>One concern is if add wait in snd_dmaengine_pcm_close(), which wil cause
> >>>the snd_pcm_release is bound with dmaengine, when there is error in dma
> >>>and no callback be called. Then the snd_pcm_release will not be released.
> >>
> >>The sync() function will only wait if there is a callback scheduled,
> >>if there is non scheduled it will return immediately.
> >
> >
> >Do you have other method to resolve this issue? or simple method?
> 
> No, this is the way to go. You have two asynchronous processes and
> you want to get deterministic execution ordering between the two you
> need to add a synchronization primitive.
> 
> - Lars
> 
Thanks your advice. It is workable. But need to add new api in dmaengine.h

I think about another method, can we add spin_lock in 
dmaengine_pcm_dma_complete() and snd_dmaengine_pcm_close()? and add a flag
for pcm_close, if pcm closed, then just do nothing in dma complete.
How do you think for this?


best regards
wang shengjiu