linux-kernel - Re: [PATCH v2] staging: tidspbridge: protect dmm

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Date:	Wed, 29 Dec 2010 11:51:34 +0200
From:	Ohad Ben-Cohen <ohad@...ery.com>
To:	Felipe Contreras <felipe.contreras@...il.com>
Cc:	Felipe Contreras <felipe.contreras@...ia.com>,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	greg@...ah.com, omar.ramirez@...com, fernando.lugo@...com,
	nm@...com, ameya.palande@...ia.com, h-kanigeri2@...com
Subject: Re: [PATCH v2] staging: tidspbridge: protect dmm_map properly

On Wed, Dec 29, 2010 at 11:42 AM, Felipe Contreras
<felipe.contreras@...il.com> wrote:
> On Tue, Dec 28, 2010 at 4:48 PM, Ohad Ben-Cohen <ohad@...ery.com> wrote:
>> On Tue, Dec 28, 2010 at 2:12 PM, Felipe Contreras
>> <felipe.contreras@...il.com> wrote:
>>> On Tue, Dec 28, 2010 at 12:56 PM, Ohad Ben-Cohen <ohad@...ery.com> wrote:
>>>> I still don't know how exactly you triggered the bug: is gst-dsp
>>>> multithreaded ? and one of its threads invoked proc_un_map() while
>>>> another thread called proc_begin_dma() ?
>>>
>>> I haven't investigated why that happens
>>
>> Btw, I still think you should look into this.
>>
>> The kernel panic will be solved, but you may still have a race there
>> that can lead to data corruption: if proc_un_map will be fast enough,
>> it will acquire the proc_lock mutex before proc_begin_dma(), and then
>> you will miss a cache operation.
>
> Aquiring the lock is the first thing done; if proc_un_map() aquires
> the lock first, it's because it was run first

Not true.

Again, we have two threads:

T1 - called proc_begin_dma()

T2 - called proc_un_map()

Let's say T1 called proc_begin_dma() first, but it still didn't
acquire the lock.

At this point the scheduler let T2 run. It calls proc_un_map() and
acquire the lock, and free all map_obj's.

Then the scheduler let T1 continue to execute, but it can't find any
map_obj, and the required cache operation is not performed.

Of course the kernel has nothing to do with this, and don't care really.

That's why I said it's a user space race - you need to make sure T2
will not call proc_un_map() before T1 (or any other thread you have)
completed executing all DMA operations (that you care about).

>, and thus a problem for
> user-space. If user-space wants the cache operation, it must run
> proc_begin_dma() first, there's nothing kernel-space can do to fix
> that.
>
> --
> Felipe Contreras
>
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