locking/mutex: Don't hog RCU read lock while optimistically spinning

There's no reason to hold an RCU read lock the entire time while
optimistically spinning for a mutex lock. This can needlessly lengthen
RCU grace periods and slow down synchronize_rcu() when it doesn't brute
force the RCU grace period via rcupdate.rcu_expedited=1.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Ruchit <ruchitmarathe@gmail.com>

kernel/locking/mutex.c

@@ -426,21 +426,31 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 {
 	bool ret = true;
 
-	rcu_read_lock();
-	while (__mutex_owner(lock) == owner) {
+	for (;;) {
+		unsigned int cpu;
+		bool same_owner;
+
 		/*
-		 * Ensure we emit the owner->on_cpu, dereference _after_
-		 * checking lock->owner still matches owner. If that fails,
+		 * Ensure lock->owner still matches owner. If that fails,
 		 * owner might point to freed memory. If it still matches,
 		 * the rcu_read_lock() ensures the memory stays valid.
 		 */
-		barrier();
+		rcu_read_lock();
+		same_owner = __mutex_owner(lock) == owner;
+		if (same_owner) {
+			ret = owner->on_cpu;
+			if (ret)
+				cpu = task_cpu(owner);
+		}
+		rcu_read_unlock();
+
+		if (!ret || !same_owner)
+			break;
 
 		/*
 		 * Use vcpu_is_preempted to detect lock holder preemption issue.
 		 */
-		if (!owner->on_cpu || need_resched() ||
-				vcpu_is_preempted(task_cpu(owner))) {
+		if (need_resched() || vcpu_is_preempted(cpu)) {
 			ret = false;
 			break;
 		}
@@ -452,7 +462,6 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 
 		cpu_relax();
 	}
-	rcu_read_unlock();
 
 	return ret;
 }