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

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Message-ID: <AANLkTimMe97QECEJ+T6Eh7SFKqOi8xo08ZB+S+8Yzuww@mail.gmail.com>
Date:	Wed, 29 Dec 2010 11:39:05 +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 2:37 PM, Ohad Ben-Cohen <ohad@...ery.com> wrote:
> On Tue, Dec 28, 2010 at 2:24 PM, Felipe Contreras
> <felipe.contreras@...il.com> wrote:
>> user-space crashed, not kernel-space; the code would continue to run
>> and eventually release the lock.
>
> So you'll have to be more specific about the scenario you are describing.
>
> If there's a user thread that is still running the proc_*_dma()
> function, and we agree that this thread keeps running until completion
> and then returns to user space, what's the problem ?

The problem is if the user-space process crashes exactly in the middle
of it, *before* completing. With locks there's no problem, as
proc_un_map() would wait for the lock in my patch. In your patch it
would not wait, just return -EBUSY.

> If that user thread will crash, drv_remove_all_resources() will clean
> up all map_obj's.

Not if a proc_*_dma() is still running.

There's a very narrow timing window where this could happen, but it is there.

-- 
Felipe Contreras
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