linux-kernel - Re: [PATCH] scsi: target: Save a few cycles in 'transport_lookup_[cmd|tmr]

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Date:   Fri, 12 Aug 2022 06:52:02 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     martin.petersen@...cle.com
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] scsi: target: Save a few cycles in
 'transport_lookup_[cmd|tmr]_lun()'

Le 03/11/2021 à 22:24, 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>
> ---
>   drivers/target/target_core_device.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/target/target_core_device.c b/drivers/target/target_core_device.c
> index 44bb380e7390..bfd5d5606522 100644
> --- a/drivers/target/target_core_device.c
> +++ b/drivers/target/target_core_device.c
> @@ -77,7 +77,7 @@ transport_lookup_cmd_lun(struct se_cmd *se_cmd)
>   
>   		se_lun = rcu_dereference(deve->se_lun);
>   
> -		if (!percpu_ref_tryget_live(&se_lun->lun_ref)) {
> +		if (!percpu_ref_tryget_live_rcu(&se_lun->lun_ref)) {
>   			se_lun = NULL;
>   			goto out_unlock;
>   		}
> @@ -154,7 +154,7 @@ int transport_lookup_tmr_lun(struct se_cmd *se_cmd)
>   	if (deve) {
>   		se_lun = rcu_dereference(deve->se_lun);
>   
> -		if (!percpu_ref_tryget_live(&se_lun->lun_ref)) {
> +		if (!percpu_ref_tryget_live_rcu(&se_lun->lun_ref)) {
>   			se_lun = NULL;
>   			goto out_unlock;
>   		}

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