Patch "locking/ww_mutex: Report recursive ww_mutex locking early" has been added to the 4.6-stable tree

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

locking/ww_mutex: Report recursive ww_mutex locking early

to the 4.6-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:
locking-ww_mutex-report-recursive-ww_mutex-locking-early.patch
and it can be found in the queue-4.6 subdirectory.

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

From: Chris Wilson <[email protected]>
Date: Thu, 26 May 2016 21:08:17 +0100
Subject: locking/ww_mutex: Report recursive ww_mutex locking early

From: Chris Wilson <[email protected]>

commit 0422e83d84ae24b933e4b0d4c1e0f0b4ae8a0a3b upstream.

Recursive locking for ww_mutexes was originally conceived as an exception. However, it is heavily used by the DRM atomic modesetting code. Currently, the recursive deadlock is checked after we have queued up for a busy-spin and as we never release the lock, we spin until kicked, whereupon the deadlock is discovered and reported.

A simple solution for the now common problem is to move the recursive deadlock discovery to the first action when taking the ww_mutex.

---
 kernel/locking/mutex.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

@@ -486,9 +486,6 @@ __ww_mutex_lock_check_stamp(struct mutex
 	if (!hold_ctx)
 		return 0;
 
-	if (unlikely(ctx == hold_ctx))
-		return -EALREADY;
-
 	if (ctx->stamp - hold_ctx->stamp <= LONG_MAX &&
 	    (ctx->stamp != hold_ctx->stamp || ctx > hold_ctx)) {
 #ifdef CONFIG_DEBUG_MUTEXES

@@ -514,6 +511,12 @@ __mutex_lock_common(struct mutex *lock,
 	unsigned long flags;
 	int ret;
 
+	if (use_ww_ctx) {
+		struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
+		if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
+			return -EALREADY;
+	}
+
 	preempt_disable();
 	mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
 

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

queue-4.6/locking-ww_mutex-report-recursive-ww_mutex-locking-early.patch