Patch "locking/mutex: Fix HANDOFF condition" has been added to the 5.14-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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

    locking/mutex: Fix HANDOFF condition

to the 5.14-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-mutex-fix-handoff-condition.patch
and it can be found in the queue-5.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 345bc2ed19e71b5a709c0f8096a51b3a7dce2816
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date:   Wed Jun 30 17:35:18 2021 +0200

    locking/mutex: Fix HANDOFF condition
    
    [ Upstream commit 048661a1f963e9517630f080687d48af79ed784c ]
    
    Yanfei reported that setting HANDOFF should not depend on recomputing
    @first, only on @first state. Which would then give:
    
      if (ww_ctx || !first)
        first = __mutex_waiter_is_first(lock, &waiter);
      if (first)
        __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
    
    But because 'ww_ctx || !first' is basically 'always' and the test for
    first is relatively cheap, omit that first branch entirely.
    
    Reported-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Reviewed-by: Waiman Long <longman@xxxxxxxxxx>
    Reviewed-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20210630154114.896786297@xxxxxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index d2df5e68b503..fb30e1436dfb 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -928,7 +928,6 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 		    struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
 {
 	struct mutex_waiter waiter;
-	bool first = false;
 	struct ww_mutex *ww;
 	int ret;
 
@@ -1007,6 +1006,8 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
 	set_current_state(state);
 	for (;;) {
+		bool first;
+
 		/*
 		 * Once we hold wait_lock, we're serialized against
 		 * mutex_unlock() handing the lock off to us, do a trylock
@@ -1035,15 +1036,9 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 		spin_unlock(&lock->wait_lock);
 		schedule_preempt_disabled();
 
-		/*
-		 * ww_mutex needs to always recheck its position since its waiter
-		 * list is not FIFO ordered.
-		 */
-		if (ww_ctx || !first) {
-			first = __mutex_waiter_is_first(lock, &waiter);
-			if (first)
-				__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
-		}
+		first = __mutex_waiter_is_first(lock, &waiter);
+		if (first)
+			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
 
 		set_current_state(state);
 		/*