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

[Lars-Peter Clausen]

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.
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.