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
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