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

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Date:	Tue, 28 Dec 2010 14:18:02 +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 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...

>> 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 ?

> Because:
>  1) Your patch changes 114 lines, mine 18
>  2) It hasn't been reviewed, nor tested by other people
>  3) At least I see a potential issue
>  4) 2.6.37 is imminent
>
> IMO one patch has chances going into 2.6.37, the other one doesn't. I
> don't see the problem of pushing my patch to 2.6.37, and once your
> patch has been properly reviewed, and tested, put it for the 2.6.38
> merge window. Anyway, it's looking more and more that this patch would
> not be ack'ed in time, so I guess we would have to wait to see what
> other people (Omar?) say, which would probably be 2.6.38 timeline.

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...
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