linux-kernel - Re: dmatest regression in 3.10-rc1

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Date:	Fri, 17 May 2013 15:18:57 +0100
From:	Will Deacon <will.deacon@....com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	"djbw@...com" <djbw@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"andriy.shevchenko@...ux.intel.com" 
	<andriy.shevchenko@...ux.jf.intel.com>,
	"viresh.kumar@...aro.org" <viresh.kumar@...aro.org>
Subject: Re: dmatest regression in 3.10-rc1

Hi Vinod,

Thanks for the reply.

On Fri, May 17, 2013 at 01:34:23PM +0100, Vinod Koul wrote:
> On Thu, May 16, 2013 at 04:35:53PM +0100, Will Deacon wrote:
> > Right, so I think I understand what's causing this, but I'll leave it to
> > Andriy to suggest a fix. The problem comes about because the dmatest
> > module is now driven from debugfs, making it possible to unload the module
> > whilst a test run is in progress. In this case:
> > 
> > 	- The DMA threads will return from wait_event_freezable_timeout(...)
> > 	  due to kthread_should_stop() returning true, and subsequently
> > 	  report failure because done.done is false.
> > 
> > 	- The DMA engines may not be idle, so the asynchronous callback can
> > 	  be invoked after we've started cleaning up, explaining the NULL
> > 	  dereference I'm seeing.
> > 
> > The solutions are either fixing the module exit code to cope with concurrent
> > DMA transfers or to revert 77101ce578bb and not allow the channel threads to
> > return mid-transfer.
> We need to properly abort the channels on removal. This is already handled in
> the code but the kthread_stop is called after the transactions are aborted. It
> should be the other way round. Can you try with below patch

Unfortunately, I can trigger the exact same panic with this patch applied.

Isn't there a race between terminating the dmaengine transfers
(dmaengine_terminate_all) and killing the test threads (kthread_stop) where
a new transfer could be kicked off by dmatest_func?

Will
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