linux-kernel - Re: [PATCH next v2 1/5] locking/osq_lock: Defer clearing node->locked until the slow osq

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Date: Sun, 31 Dec 2023 23:08:07 -0500
From: Waiman Long <longman@...hat.com>
To: David Laight <David.Laight@...LAB.COM>,
 "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
 "'peterz@...radead.org'" <peterz@...radead.org>
Cc: "'mingo@...hat.com'" <mingo@...hat.com>,
 "'will@...nel.org'" <will@...nel.org>,
 "'boqun.feng@...il.com'" <boqun.feng@...il.com>,
 'Linus Torvalds' <torvalds@...ux-foundation.org>,
 "'virtualization@...ts.linux-foundation.org'"
 <virtualization@...ts.linux-foundation.org>,
 'Zeng Heng' <zengheng4@...wei.com>
Subject: Re: [PATCH next v2 1/5] locking/osq_lock: Defer clearing node->locked
 until the slow osq_lock() path.

On 12/31/23 16:51, David Laight wrote:
> Since node->locked cannot be set before the assignment to prev->next
> it is save to clear it in the slow path.
>
> Signed-off-by: David Laight <david.laight@...lab.com>
> ---
>   kernel/locking/osq_lock.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 75a6f6133866..e0bc74d85a76 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -97,7 +97,6 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   	int curr = encode_cpu(smp_processor_id());
>   	int old;
>   
> -	node->locked = 0;
>   	node->next = NULL;
>   	node->cpu = curr;
>   
> @@ -111,6 +110,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   	if (old == OSQ_UNLOCKED_VAL)
>   		return true;
>   
> +	node->locked = 0;
>   	prev = decode_cpu(old);
>   	node->prev = prev;
>   

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