From mboxrd@z Thu Jan  1 00:00:00 1970
From: peterz@infradead.org (Peter Zijlstra)
Date: Thu, 5 Apr 2018 19:19:12 +0200
Subject: [PATCH 03/10] locking/qspinlock: Kill cmpxchg loop when claiming
 lock from head of queue
In-Reply-To: <1522947547-24081-4-git-send-email-will.deacon@arm.com>
References: <1522947547-24081-1-git-send-email-will.deacon@arm.com>
 <1522947547-24081-4-git-send-email-will.deacon@arm.com>
Message-ID: <20180405171912.GE4082@hirez.programming.kicks-ass.net>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Thu, Apr 05, 2018 at 05:59:00PM +0100, Will Deacon wrote:
> +
> +	/* 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.
>  		 */
>  		old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
>  		if (old == val)
> +			goto release; /* No contention */
>  	}

--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -464,8 +464,7 @@ void queued_spin_lock_slowpath(struct qs
 		 * The smp_cond_load_acquire() call above has provided the
 		 * necessary acquire semantics required for locking.
 		 */
-		old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
-		if (old == val)
+		if (atomic_try_cmpxchg_release(&lock->val, &val, _Q_LOCKED_VAL))
 			goto release; /* No contention */
 	}

Does that also work for you? It would generate slightly better code for
x86 (not that it would matter much on this path).