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

[Lars-Peter Clausen]

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