linux-kernel - Re: [PATCH 2/2] dma:of: Use a mutex to protect the of_dma

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Message-ID: <517124D6.7090008@metafoo.de>
Date:	Fri, 19 Apr 2013 13:04:54 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Vinod Koul <vinod.koul@...el.com>, Jon Hunter <jon-hunter@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] dma:of: Use a mutex to protect the of_dma_list

On 04/19/2013 12:13 PM, Arnd Bergmann wrote:
> On Friday 19 April 2013, Lars-Peter Clausen wrote:
> 
>> This is rather ugly and unnecessary and non of the current users of
>> of_dma_controller_free() check it's return value anyway. Instead protect the
>> list by a mutex. The mutex will be held as long as a request operation is in
>> progress. So if of_dma_controller_free() is called while a request operation is
>> in progress it will be put to sleep and only wake up once the request operation
>> has finished.
>>
>> This means that it is no longer possible to register or unregister OF DMA
>> controllers from a context where it's not possible to sleep. But I doubt that
>> we'll ever need this.
>>
>> Also rename of_dma_get_controller back to of_dma_find_controller.
>>
>> Signed-off-by: Lars-Peter Clausen <lars@...afoo.de>
> 
> I guess we could also fix it by turning the reference count into a proper
> kref with kref_put calling the destructor, but your solution seems to solve
> the problem just as well.

Well, It's not only about the of_dma struct, e.g. the of_dma's of_dma_data
field usually points to some driver allocated data. If we do not block in
of_dma_controller_free() this data might be freed while the controller is
still in use.
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