linux-kernel - Re: [PATCH] locking/osq_lock: Use atomic_try_cmpxchg_release() in osq

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Message-ID: <905c96ba-c11f-4829-b8a4-c67edc511ae9@redhat.com>
Date: Wed, 2 Oct 2024 11:47:30 -0400
From: Waiman Long <longman@...hat.com>
To: Uros Bizjak <ubizjak@...il.com>, linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] locking/osq_lock: Use atomic_try_cmpxchg_release() in
 osq_unlock()

On 10/1/24 07:45, Uros Bizjak wrote:
> Replace this pattern in osq_unlock():
>
>      atomic_cmpxchg(*ptr, old, new) == old
>
> ... with the simpler and faster:
>
>      atomic_try_cmpxchg(*ptr, &old, new)
>
> The x86 CMPXCHG instruction returns success in the ZF flag,
> so this change saves a compare after the CMPXCHG.  The code
> in the fast path of osq_unlock() improves from:
>
>   11b:	31 c9                	xor    %ecx,%ecx
>   11d:	8d 50 01             	lea    0x1(%rax),%edx
>   120:	89 d0                	mov    %edx,%eax
>   122:	f0 0f b1 0f          	lock cmpxchg %ecx,(%rdi)
>   126:	39 c2                	cmp    %eax,%edx
>   128:	75 05                	jne    12f <...>
>
> to:
>
>   12b:	31 d2                	xor    %edx,%edx
>   12d:	83 c0 01             	add    $0x1,%eax
>   130:	f0 0f b1 17          	lock cmpxchg %edx,(%rdi)
>   134:	75 05                	jne    13b <...>
>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> ---
>   kernel/locking/osq_lock.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 75a6f6133866..b4233dc2c2b0 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -215,8 +215,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
>   	/*
>   	 * Fast path for the uncontended case.
>   	 */
> -	if (likely(atomic_cmpxchg_release(&lock->tail, curr,
> -					  OSQ_UNLOCKED_VAL) == curr))
> +	if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL))
>   		return;
>   
>   	/*

LGTM

Acked-by: Waiman Long <longman@...hat.com>