linux-kernel - Re: [PATCH] aio: Save a few cycles in 'lookup

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Message-ID: <a8666743-4dc5-79b8-56c7-23c05fc88d66@wanadoo.fr>
Date:   Fri, 12 Aug 2022 06:54:09 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     bcrl@...ck.org, viro@...iv.linux.org.uk
Cc:     linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] aio: Save a few cycles in 'lookup_ioctx()'

Le 03/11/2021 à 22:31, Christophe JAILLET a écrit :
> Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
> save a few cycles when it is known that the rcu lock is already
> taken/released.
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
>   fs/aio.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 9c81cf611d65..d189ea13e10a 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1062,7 +1062,7 @@ static struct kioctx *lookup_ioctx(unsigned long ctx_id)
>   	id = array_index_nospec(id, table->nr);
>   	ctx = rcu_dereference(table->table[id]);
>   	if (ctx && ctx->user_id == ctx_id) {
> -		if (percpu_ref_tryget_live(&ctx->users))
> +		if (percpu_ref_tryget_live_rcu(&ctx->users))
>   			ret = ctx;
>   	}
>   out:


Hi,
gentle reminder.

Is this patch useful?
When I first posted it, percpu_ref_tryget_live_rcu() was really new.
Now it is part of linux since 5.16.

Saving a few cycles in a function with "lookup" in its name looks always 
good to me.

CJ