linux-kernel - Re: [PATCH v2] dmaengine: edma: Implement device

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Date:	Thu, 11 Feb 2016 13:12:20 +0200
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Lars-Peter Clausen <lars@...afoo.de>, <vinod.koul@...el.com>
CC:	<linux-kernel@...r.kernel.org>, <dmaengine@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <nsekhar@...com>,
	<linux-omap@...r.kernel.org>
Subject: Re: [PATCH v2] dmaengine: edma: Implement device_synchronize callback

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@...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

View attachment "0001-dmaengine-virt-dma-Support-for-ignoring-callback-aft.patch" of type "text/x-patch" (4081 bytes)

View attachment "0002-dmaengine-omap-dma-Prevent-race-between-vchan_comple.patch" of type "text/x-patch" (1215 bytes)

View attachment "0003-dmaengine-edma-Prevent-race-between-vchan_complete-a.patch" of type "text/x-patch" (1231 bytes)