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Date:   Fri, 6 Oct 2023 19:15:10 +0200
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Subject: Re: [PATCH] fuse: remove unneeded lock which protecting update of
 congestion_threshold
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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>
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 <a66f94b8-4330-4e0d-589c-a031a5b3802c@huaweicloud.com>
From:   Bernd Schubert <bernd.schubert@fastmail.fm>
In-Reply-To: <a66f94b8-4330-4e0d-589c-a031a5b3802c@huaweicloud.com>
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On 9/27/23 05:04, Kemeng Shi wrote:
> 
> 
> 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!

Sounds good to me, thanks for reading the document carefully!


Bernd