[PATCH] drm: Fix locking for sysfs dpms file

[Jens Axboe]

On 09/29/2015 01:56 AM, Daniel Vetter wrote:
> With atomic drivers we need to make sure that (at least in general)
> property reads hold the right locks. But the legacy dpms property is
> special and can be read locklessly. Since userspace loves to just
> randomly look at that all the time (like with "status") do that.
>
> To make it clear that we play tricks use the READ_ONCE compiler
> barrier (and also for paranoia).
>
> Note that there's not really anything bad going on since even with the
> new atomic paths we eventually end up not chasing any pointers (and
> hence possibly freed memory and other fun stuff). The locking WARNING
> has been added in
>
> commit 88a48e297b3a3bac6022c03babfb038f1a886cea
> Author: Rob Clark <robdclark at gmail.com>
> Date:   Thu Dec 18 16:01:50 2014 -0500
>
>      drm: add atomic properties
>
> but since drivers are converting not everyone will have seen this from
> the start.
>
> Jens reported this and submitted a patch to just grab the
> mode_config.connection_mutex, but we can do a bit better.
>
> v2: Remove unused variables I failed to git add for real.
>
> Reported-by: Jens Axboe <axboe at fb.com>
> Cc: Jens Axboe <axboe at fb.com>
> Cc: Rob Clark <robdclark at gmail.com>
> Cc: stable at vger.kernel.org
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>

Works for me, thanks Daniel.

Tested-by: Jens Axboe <axboe at fb.com>

-- 
Jens Axboe