[PATCH] drm: Fix locking for sysfs dpms file

[Daniel Vetter]

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.

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 | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
index f08873f6489c..63adcd79226f 100644
--- a/drivers/gpu/drm/drm_sysfs.c
+++ b/drivers/gpu/drm/drm_sysfs.c
@@ -231,17 +231,13 @@ static ssize_t dpms_show(struct device *device,
 {
 	struct drm_connector *connector = to_drm_connector(device);
 	struct drm_device *dev = connector->dev;
-	uint64_t dpms_status;
+	int dpms;
 	int ret;
 
-	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