[PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[Kemeng Shi]

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;
-- 
2.30.0

Re: [PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[Bernd Schubert]

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

Re: [PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[Kemeng Shi]

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
> 

Re: [PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[Bernd Schubert]

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!

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
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.

Thanks,
Bernd

Re: [PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[Kemeng Shi]

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
> 
> 
> 
> 

Re: [PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold

[Bernd Schubert]

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