Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <f5210302-13b4-a9a2-8b69-469f7d68e370@wanadoo.fr>
Date:   Fri, 12 Aug 2022 06:52:02 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.11.0
Subject: Re: [PATCH] scsi: target: Save a few cycles in
 'transport_lookup_[cmd|tmr]_lun()'
Content-Language: en-US
To:     martin.petersen@oracle.com
Cc:     linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-janitors@vger.kernel.org
References: <e4a21bc607c39935cb98d4825cd63ba349820550.1635974637.git.christophe.jaillet@wanadoo.fr>
From:   Christophe JAILLET <christophe.jaillet@wanadoo.fr>
In-Reply-To: <e4a21bc607c39935cb98d4825cd63ba349820550.1635974637.git.christophe.jaillet@wanadoo.fr>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

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@wanadoo.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