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

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Date:   Mon, 15 Aug 2022 14:10:49 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     bcrl@...ck.org, viro@...iv.linux.org.uk, 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()'

Christophe JAILLET <christophe.jaillet@...adoo.fr> writes:

> 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.

The patch looks fine to me.

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>