[RFC][PATCH 1/4] locking/mutex: Use try_cmpxchg()

[Peter Zijlstra]

For simpler and better code.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/locking/mutex.c |   27 ++++++---------------------
 1 file changed, 6 insertions(+), 21 deletions(-)

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -100,7 +100,7 @@ static inline struct task_struct *__mute
 
 	owner = atomic_long_read(&lock->owner);
 	for (;;) { /* must loop, can race against a flag */
-		unsigned long old, flags = __owner_flags(owner);
+		unsigned long flags = __owner_flags(owner);
 		unsigned long task = owner & ~MUTEX_FLAGS;
 
 		if (task) {
@@ -124,11 +124,8 @@ static inline struct task_struct *__mute
 		 */
 		flags &= ~MUTEX_FLAG_HANDOFF;
 
-		old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);
-		if (old == owner)
+		if (atomic_long_try_cmpxchg_acquire(&lock->owner, &owner, curr | flags))
 			return NULL;
-
-		owner = old;
 	}
 
 	return __owner_task(owner);
@@ -168,10 +165,7 @@ static __always_inline bool __mutex_unlo
 {
 	unsigned long curr = (unsigned long)current;
 
-	if (atomic_long_cmpxchg_release(&lock->owner, curr, 0UL) == curr)
-		return true;
-
-	return false;
+	return atomic_long_try_cmpxchg_release(&lock->owner, &curr, 0UL);
 }
 #endif
 
@@ -226,7 +220,7 @@ static void __mutex_handoff(struct mutex
 	unsigned long owner = atomic_long_read(&lock->owner);
 
 	for (;;) {
-		unsigned long old, new;
+		unsigned long new;
 
 #ifdef CONFIG_DEBUG_MUTEXES
 		DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
@@ -238,11 +232,8 @@ static void __mutex_handoff(struct mutex
 		if (task)
 			new |= MUTEX_FLAG_PICKUP;
 
-		old = atomic_long_cmpxchg_release(&lock->owner, owner, new);
-		if (old == owner)
+		if (atomic_long_try_cmpxchg_release(&lock->owner, &owner, new))
 			break;
-
-		owner = old;
 	}
 }
 
@@ -1237,8 +1228,6 @@ static noinline void __sched __mutex_unl
 	 */
 	owner = atomic_long_read(&lock->owner);
 	for (;;) {
-		unsigned long old;
-
 #ifdef CONFIG_DEBUG_MUTEXES
 		DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
 		DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);
@@ -1247,16 +1236,12 @@ static noinline void __sched __mutex_unl
 		if (owner & MUTEX_FLAG_HANDOFF)
 			break;
 
-		old = atomic_long_cmpxchg_release(&lock->owner, owner,
-						  __owner_flags(owner));
-		if (old == owner) {
+		if (atomic_long_try_cmpxchg_release(&lock->owner, &owner, __owner_flags(owner))) {
 			if (owner & MUTEX_FLAG_WAITERS)
 				break;
 
 			return;
 		}
-
-		owner = old;
 	}
 
 	spin_lock(&lock->wait_lock);