linux-kernel - Locking question for DRM

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Message-ID: <20100423112222.67cfcbb2@lxorguk.ukuu.org.uk>
Date:	Fri, 23 Apr 2010 11:22:22 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Dave Airlie <airlied@...ux.ie>, dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: Locking question for DRM

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 ?

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 ?

Alan
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