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Date: Thu, 21 Mar 2024 19:52:21 -0400
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Subject: Re: [PATCH] locking/qspinlock: Use atomic_try_cmpxchg_relaxed() in
 xchg_tail()
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To: Uros Bizjak <ubizjak@gmail.com>, linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
 Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>
References: <20240321195309.484275-1-ubizjak@gmail.com>
From: Waiman Long <longman@redhat.com>
In-Reply-To: <20240321195309.484275-1-ubizjak@gmail.com>
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On 3/21/24 15:52, Uros Bizjak wrote:
> Use atomic_try_cmpxchg_relaxed(*ptr, &old, new) instead of
> atomic_cmpxchg_relaxed (*ptr, old, new) == old in xchg_tail().
> x86 CMPXCHG instruction returns success in ZF flag,
> so this change saves a compare after cmpxchg.
>
> No functional change intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> ---
>   kernel/locking/qspinlock.c | 13 +++++--------
>   1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index ebe6b8ec7cb3..1df5fef8a656 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -220,21 +220,18 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
>    */
>   static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
>   {
> -	u32 old, new, val = atomic_read(&lock->val);
> +	u32 old, new;
>   
> -	for (;;) {
> -		new = (val & _Q_LOCKED_PENDING_MASK) | tail;
> +	old = atomic_read(&lock->val);
> +	do {
> +		new = (old & _Q_LOCKED_PENDING_MASK) | tail;
>   		/*
>   		 * We can use relaxed semantics since the caller ensures that
>   		 * the MCS node is properly initialized before updating the
>   		 * tail.
>   		 */
> -		old = atomic_cmpxchg_relaxed(&lock->val, val, new);
> -		if (old == val)
> -			break;
> +	} while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new));
>   
> -		val = old;
> -	}
>   	return old;
>   }
>   #endif /* _Q_PENDING_BITS == 8 */

LGTM, note that this xchg_tail() variant is not used in all the distros 
that I am aware of as it requires NR_CPUS >= 16k.

Reviewed-by: Waiman Long <longman@redhat.com>