linux-kernel - Re: Locking question for DRM

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Date:	Fri, 23 Apr 2010 20:44:38 +1000
From:	Dave Airlie <airlied@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: Locking question for DRM

On Fri, Apr 23, 2010 at 8:37 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> > and I can't see what makes this safe if the drm_release for the master
>> > and a client occur at the same time ?
>>
>> lock_kernel in drm_release. We probably need to clean that up.
>
> I don't think that works. drm_open_helper doesn't appear to be under the
> BKL merely the struct mutex.

It blocks the case you specified of two releases happening together.

>
>> > The setmaster/dropmaster ioctl seems similar - the various conditional
>> > checks are not protected from parallel changes occuring during their
>> > execution.
>> >
>> > Is this a bug or is something clever afoot ?
>>
>> These ioctls are also under the BKL.
>
> But setmaster can sleep so the BKL is dropped on contention of the
> struct_mutex, ditto dropmaster

they should only sleep in the mutex lock nuless the driver callback is
allocating memory. but yeah its a bit of a mess.

Dave.
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