Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:4 as permitted sender) client-ip=2620:137:e000::3:4;
Feedback-ID: id8a24192:Fastmail
Message-ID: <9a5d4c82-1ab3-e96d-98bb-369acc8404d1@fastmail.fm>
Date:   Sat, 16 Sep 2023 13:06:44 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.15.1
Subject: Re: [PATCH] fuse: remove unneeded lock which protecting update of
 congestion_threshold
To:     Kemeng Shi <shikemeng@huaweicloud.com>, miklos@szeredi.hu,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20230914154553.71939-1-shikemeng@huaweicloud.com>
Content-Language: en-US, de-DE
From:   Bernd Schubert <bernd.schubert@fastmail.fm>
In-Reply-To: <20230914154553.71939-1-shikemeng@huaweicloud.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 9/14/23 17:45, Kemeng Shi wrote:
> Commit 670d21c6e17f6 ("fuse: remove reliance on bdi congestion") change how
> congestion_threshold is used and lock in
> fuse_conn_congestion_threshold_write is not needed anymore.
> 1. Access to supe_block is removed along with removing of bdi congestion.
> Then down_read(&fc->killsb) which protecting access to super_block is no
> needed.
> 2. Compare num_background and congestion_threshold without holding
> bg_lock. Then there is no need to hold bg_lock to update
> congestion_threshold.
> 
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> ---
>   fs/fuse/control.c | 4 ----
>   1 file changed, 4 deletions(-)
> 
> diff --git a/fs/fuse/control.c b/fs/fuse/control.c
> index 247ef4f76761..c5d7bf80efed 100644
> --- a/fs/fuse/control.c
> +++ b/fs/fuse/control.c
> @@ -174,11 +174,7 @@ static ssize_t fuse_conn_congestion_threshold_write(struct file *file,
>   	if (!fc)
>   		goto out;
>   
> -	down_read(&fc->killsb);
> -	spin_lock(&fc->bg_lock);
>   	fc->congestion_threshold = val;
> -	spin_unlock(&fc->bg_lock);
> -	up_read(&fc->killsb);
>   	fuse_conn_put(fc);
>   out:
>   	return ret;

Yeah, I don't see readers holding any of these locks.
I just wonder if it wouldn't be better to use WRITE_ONCE to ensure a 
single atomic operation to store the value.


Thanks,
Bernd