Merge branch 'locking/urgent' into locking/core, to pick up dependent fixes

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 66 insertions, 2 deletions

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index d8d7a15..6e6cab7 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -65,8 +65,72 @@ static inline void clear_rt_mutex_waiters(struct rt_mutex *lock)
 
 static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
 {
-	if (!rt_mutex_has_waiters(lock))
-		clear_rt_mutex_waiters(lock);
+	unsigned long owner, *p = (unsigned long *) &lock->owner;
+
+	if (rt_mutex_has_waiters(lock))
+		return;
+
+	/*
+	 * The rbtree has no waiters enqueued, now make sure that the
+	 * lock->owner still has the waiters bit set, otherwise the
+	 * following can happen:
+	 *
+	 * CPU 0	CPU 1		CPU2
+	 * l->owner=T1
+	 *		rt_mutex_lock(l)
+	 *		lock(l->lock)
+	 *		l->owner = T1 | HAS_WAITERS;
+	 *		enqueue(T2)
+	 *		boost()
+	 *		  unlock(l->lock)
+	 *		block()
+	 *
+	 *				rt_mutex_lock(l)
+	 *				lock(l->lock)
+	 *				l->owner = T1 | HAS_WAITERS;
+	 *				enqueue(T3)
+	 *				boost()
+	 *				  unlock(l->lock)
+	 *				block()
+	 *		signal(->T2)	signal(->T3)
+	 *		lock(l->lock)
+	 *		dequeue(T2)
+	 *		deboost()
+	 *		  unlock(l->lock)
+	 *				lock(l->lock)
+	 *				dequeue(T3)
+	 *				 ==> wait list is empty
+	 *				deboost()
+	 *				 unlock(l->lock)
+	 *		lock(l->lock)
+	 *		fixup_rt_mutex_waiters()
+	 *		  if (wait_list_empty(l) {
+	 *		    l->owner = owner
+	 *		    owner = l->owner & ~HAS_WAITERS;
+	 *		      ==> l->owner = T1
+	 *		  }
+	 *				lock(l->lock)
+	 * rt_mutex_unlock(l)		fixup_rt_mutex_waiters()
+	 *				  if (wait_list_empty(l) {
+	 *				    owner = l->owner & ~HAS_WAITERS;
+	 * cmpxchg(l->owner, T1, NULL)
+	 *  ===> Success (l->owner = NULL)
+	 *
+	 *				    l->owner = owner
+	 *				      ==> l->owner = T1
+	 *				  }
+	 *
+	 * With the check for the waiter bit in place T3 on CPU2 will not
+	 * overwrite. All tasks fiddling with the waiters bit are
+	 * serialized by l->lock, so nothing else can modify the waiters
+	 * bit. If the bit is set then nothing can change l->owner either
+	 * so the simple RMW is safe. The cmpxchg() will simply fail if it
+	 * happens in the middle of the RMW because the waiters bit is
+	 * still set.
+	 */
+	owner = READ_ONCE(*p);
+	if (owner & RT_MUTEX_HAS_WAITERS)
+		WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);
 }
 
 /*