sched: Simplify mutex_spin_on_owner()

It does not make sense to rcu_read_lock/unlock() in every loop
iteration while spinning on the mutex.

Move the rcu protection outside the loop. Also simplify the
return path to always check for lock->owner == NULL which
meets the requirements of both owner changed and need_resched()
caused loop exits.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1106101458350.11814@ionos
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -4306,11 +4306,8 @@ EXPORT_SYMBOL(schedule);
 
 static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
 {
-	bool ret = false;
-
-	rcu_read_lock();
 	if (lock->owner != owner)
-		goto fail;
+		return false;
 
 	/*
 	 * Ensure we emit the owner->on_cpu, dereference _after_ checking
@@ -4320,11 +4317,7 @@ static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
 	 */
 	barrier();
 
-	ret = owner->on_cpu;
-fail:
-	rcu_read_unlock();
-
-	return ret;
+	return owner->on_cpu;
 }
 
 /*
@@ -4336,21 +4329,21 @@ int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 	if (!sched_feat(OWNER_SPIN))
 		return 0;
 
+	rcu_read_lock();
 	while (owner_running(lock, owner)) {
 		if (need_resched())
-			return 0;
+			break;
 
 		arch_mutex_cpu_relax();
 	}
+	rcu_read_unlock();
 
 	/*
-	 * If the owner changed to another task there is likely
-	 * heavy contention, stop spinning.
+	 * We break out the loop above on need_resched() and when the
+	 * owner changed, which is a sign for heavy contention. Return
+	 * success only when lock->owner is NULL.
 	 */
-	if (lock->owner)
-		return 0;
-
-	return 1;
+	return lock->owner == NULL;
 }
 #endif