linux-kernel - Re: Locking question for DRM

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Date:	Fri, 23 Apr 2010 20:24:28 +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 8:22 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>
> Various bits of the DRM deal with minor->master:
>
> In the case of the open helper its protected by the struct mutex.
>
> In the release path it's protected on some paths, but not this one ...
>
>     /* if the master has gone away we can't do anything with the lock */
>        if (file_priv->minor->master)
>                drm_master_release(dev, filp);
>
> 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.

>
> 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.

So yes its nasty, and we should probably grow a minor lock to protect that.

Dave.
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