linux-kernel - Re: [PATCH 03/10] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue

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Message-ID: <20180405171912.GE4082@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Apr 2018 19:19:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        mingo@...nel.org, boqun.feng@...il.com, paulmck@...ux.vnet.ibm.com,
        catalin.marinas@....com, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH 03/10] locking/qspinlock: Kill cmpxchg loop when claiming
 lock from head of queue

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).