[PATCH] drm: Fix locking for sysfs dpms file

[Jani Nikula]

On Tue, 29 Sep 2015, Daniel Vetter <daniel.vetter at ffwll.ch> 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.
>

Reference: http://mid.gmane.org/20150928194822.GA3930@kernel.dk

> 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>
> ---
>  drivers/gpu/drm/drm_sysfs.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
> index f08873f6489c..615b7e667320 100644
> --- a/drivers/gpu/drm/drm_sysfs.c
> +++ b/drivers/gpu/drm/drm_sysfs.c
> @@ -230,18 +230,12 @@ static ssize_t dpms_show(struct device *device,
>  			   char *buf)
>  {
>  	struct drm_connector *connector = to_drm_connector(device);
> -	struct drm_device *dev = connector->dev;
> -	uint64_t dpms_status;
> -	int ret;
> +	int dpms;
>  
> -	ret = drm_object_property_get_value(&connector->base,
> -					    dev->mode_config.dpms_property,
> -					    &dpms_status);
> -	if (ret)
> -		return 0;
> +	dpms = READ_ONCE(connector->dpms);
>  
>  	return snprintf(buf, PAGE_SIZE, "%s\n",
> -			drm_get_dpms_name((int)dpms_status));
> +			drm_get_dpms_name(dpms));
>  }
>  
>  static ssize_t enabled_show(struct device *device,
> -- 
> 2.5.1
>
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-- 
Jani Nikula, Intel Open Source Technology Center