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

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Message-ID: <AANLkTinZ0Uwp8jbmsSwm6ut75A06CxrkESTRJT-GoPkP@mail.gmail.com>
Date:	Tue, 28 Dec 2010 14:24:03 +0200
From:	Felipe Contreras <felipe.contreras@...il.com>
To:	Ohad Ben-Cohen <ohad@...ery.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 Tue, Dec 28, 2010 at 2:18 PM, Ohad Ben-Cohen <ohad@...ery.com> wrote:
> On Tue, Dec 28, 2010 at 2:12 PM, Felipe Contreras
> <felipe.contreras@...il.com> wrote:
>> I haven't investigated why that happens, but kernel-space should not
>> panic regardless of what user-space does.
>
> Agree of course. But that's not what we were discussing...

Well, hopefully after applying your patch it would be easier to figure that out.

>>> Anyhow, a thread that is calling proc_*_dma() will both increase the
>>> reference count and decrease it back before going back to user space.
>>> Otherwise your patch would be problematic as well - who will unlock
>>> the mutex you take in proc_*_dma() ?
>>
>> I'm saying that user-space might crash *before* proc_*_dma() finishes,
>> before the reference count has been decreased.
>>
>> In my patch there would be no issue because proc_un_map() would wait
>> until proc_*_dma() has released the lock.
>
> But what will happen if, as you say, user-space would crash before
> proc_*_dma() has released the lock ? how could proc_un_map() run ?

user-space crashed, not kernel-space; the code would continue to run
and eventually release the lock.

> This is all good, and I have no problem with it. As I said, I don't
> resist your patch as a temporary fix. But it doesn't mean I like it...

Yeah, so the chances of getting this fixed on 2.6.37 are dimmed.

-- 
Felipe Contreras
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