[PATCH v2] dmaengine: edma: Implement device_synchronize callback

[Peter Ujfalusi]

On 02/11/2016 11:41 AM, Lars-Peter Clausen wrote:
> On 02/11/2016 10:08 AM, Peter Ujfalusi wrote:
>> We need the callback to support the dmaengine_terminate_sync().
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi at ti.com>
> 
> Looks good, but I noticed a slight race condition in
> edma_completion_handler(). You need to fetch echan->desc while holding the
> vchan.lock. Otherwise this can race against terminate_all() and the callback
> might get scheduled even though terminate_all() completed and then there is
> a race where the synchronize() operation could be called before the callback
> gets scheduled, which means it doesn't do its intended job. Highly unlikely
> to happen, but theoretically possible.

Right, actually I had (have) another series fixing the very same race in a
different way - patching the terminate_all (series attached).
We have seen race with RT kernel on uniprocessor setup.

The tasklet_kill after the terminate_all will execute the scheduled task
unconditionally, so the vchan_complete() will run after we have terminated the
channel, which might be not what we want.
I have also seen a race condition as explained in the first patch. The only
way I was able to fix that by using the attached patches. Unfortunately I can
not test RT with mainline yet, so I'm not 100% sure if by using the
dmaengine_terminate_sync() in drivers will fix the issue.

-- 
Péter
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