locking/qspinlock: Kill cmpxchg() loop when claiming lock from head of queue

When a queued locker reaches the head of the queue, it claims the lock
by setting _Q_LOCKED_VAL in the lockword. If there isn't contention, it
must also clear the tail as part of this operation so that subsequent
lockers can avoid taking the slowpath altogether.

Currently this is expressed as a cmpxchg() loop that practically only
runs up to two iterations. This is confusing to the reader and unhelpful
to the compiler. Rewrite the cmpxchg() loop without the loop, so that a
failed cmpxchg() implies that there is contention and we just need to
write to _Q_LOCKED_VAL without considering the rest of the lockword.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1524738868-31318-7-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/qspinlock.c

@@ -465,24 +465,21 @@ locked:
 	 * and nobody is pending, clear the tail code and grab the lock.
 	 * Otherwise, we only need to grab the lock.
 	 */
-	for (;;) {
-		/* In the PV case we might already have _Q_LOCKED_VAL set */
-		if ((val & _Q_TAIL_MASK) != tail || (val & _Q_PENDING_MASK)) {
-			set_locked(lock);
-			break;
-		}
+
+	/* In the PV case we might already have _Q_LOCKED_VAL set */
+	if ((val & _Q_TAIL_MASK) == tail) {
 		/*
 		 * The smp_cond_load_acquire() call above has provided the
-		 * necessary acquire semantics required for locking. At most
-		 * two iterations of this loop may be ran.
+		 * necessary acquire semantics required for locking.
 		 */
 		old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
 		if (old == val)
-			goto release;	/* No contention */
-
-		val = old;
+			goto release; /* No contention */
 	}
 
+	/* Either somebody is queued behind us or _Q_PENDING_VAL is set */
+	set_locked(lock);
+
 	/*
 	 * contended path; wait for next if not observed yet, release.
 	 */