Patch "drm: Fix locking for sysfs dpms file" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

drm: Fix locking for sysfs dpms file

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-fix-locking-for-sysfs-dpms-file.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Daniel Vetter <[email protected]>
Date: Tue, 29 Sep 2015 09:56:53 +0200
Subject: drm: Fix locking for sysfs dpms file

From: Daniel Vetter <[email protected]>

commit 621bd0f6982badd6483acb191eb7b6226a578328 upstream.

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 <[email protected]>
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/[email protected]

---
 drivers/gpu/drm/drm_sysfs.c |   12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

@@ -235,18 +235,12 @@ static ssize_t dpms_show(struct 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,

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/drm-fix-locking-for-sysfs-dpms-file.patch