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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>
 <177d891e-9258-68bb-72aa-4d4126403b7e@huaweicloud.com>
 <73e673d6-ecb8-dec9-bdc0-6dde9c4e76cb@fastmail.fm>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <a66f94b8-4330-4e0d-589c-a031a5b3802c@huaweicloud.com>
Date:   Wed, 27 Sep 2023 11:04:53 +0800
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on 9/19/2023 9:12 PM, Bernd Schubert wrote:
> 
> 
> On 9/19/23 08:11, Kemeng Shi wrote:
>>
>>
>> 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!
> 
Sorry for the dealy - it toke me some time to go through the barrier documents.
> I'm not entirely sure either, but I _think_ the compiler is free to store a 32 bit value  with multiple operations (like 2 x 16 bit). In that case a competing reading thread might read garbage...
> Although I don't see this documented here
> https://www.kernel.org/doc/Documentation/memory-barriers.txt
I found this is documented in section
"(*) For aligned memory locations whose size allows them to be accessed..."
Then WRITE_ONCE is absolutely needed now as you menthioned before.
> Though documented there is that the compile is free to optimize out the storage at all, see
> "(*) Similarly, the compiler is within its rights to omit a store entirely"
> 
> 
> Regarding READ_ONCE, I don't have a strong opinion, if the compiler makes some optimizations and the value would be wrong for a few cycles, would that matter for that variable? Unless the compiler would be really creative and the variable would get never updated... For sure READ_ONCE would be safer, but I don't know if it is needed
> SSee section
> "The compiler is within its rights to omit a load entirely if it know"
> in the document above.
I go through all examples of optimizations in document and congestion_threshold
has no same trouble descripted in document.
For specifc case you menthioned that "The compiler is within its rights to omit
a load entirely if it know". The compiler will keep the first load and only omit
successive loads from same variable in loop. As there is no repeat loading from
congestion_threshold in loop, congestion_threshold is out of this trouble.
Anyway, congestion_threshold is more like a hint and the worst case is that it is
stale for a few cycles. I prefer to keep reading congestion_threshold without
READ_ONCE and will do it in next version if it's fine to you. Thanks!
> 
> Thanks,
> Bernd
> 
> 
> 
>