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Subject: Re: [PATCH] fuse: remove unneeded lock which protecting update of
 congestion_threshold
To:     Bernd Schubert <bernd.schubert@fastmail.fm>, miklos@szeredi.hu,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20230914154553.71939-1-shikemeng@huaweicloud.com>
 <9a5d4c82-1ab3-e96d-98bb-369acc8404d1@fastmail.fm>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <177d891e-9258-68bb-72aa-4d4126403b7e@huaweicloud.com>
Date:   Tue, 19 Sep 2023 14:11:59 +0800
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on 9/16/2023 7:06 PM, Bernd Schubert wrote:
> 
> 
> 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.
Sure, WRITE_ONCE looks better. I wonder if we should use READ_ONCE from reader.
Would like to get any advice. Thanks!
> 
> 
> Thanks,
> Bernd
>