Re: [Phishing Risk] [External] [PATCH 2/3] mm/zsmalloc.c: combine two atomic ops in zs_pool_dec_isolated()

[Miaohe Lin]

On 2021/6/25 15:29, Muchun Song wrote:
> On Fri, Jun 25, 2021 at 2:32 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> On 2021/6/25 13:01, Muchun Song wrote:
>>> On Thu, Jun 24, 2021 at 8:40 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>>>
>>>> atomic_long_dec_and_test() is equivalent to atomic_long_dec() and
>>>> atomic_long_read() == 0. Use it to make code more succinct.
>>>
>>> Actually, they are not equal. atomic_long_dec_and_test implies a
>>> full memory barrier around it but atomic_long_dec and atomic_long_read
>>> don't.
>>>
>>
>> Many thanks for comment. They are indeed not completely equal as you said.
>> What I mean is they can do the same things we want in this specified context.
>> Thanks again.
> 
> I don't think so. Using individual operations can eliminate memory barriers.
> We will pay for the barrier if we use atomic_long_dec_and_test here.

The combination of atomic_long_dec and atomic_long_read usecase is rare and looks somehow
weird. I think it's worth to do this with the cost of barrier.

> 
>>
>>> That RMW operations that have a return value is equal to the following.
>>>
>>> smp_mb__before_atomic()
>>> non-RMW operations or RMW operations that have no return value
>>> smp_mb__after_atomic()
>>>
>>> Thanks.
>>>
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>> ---
>>>>  mm/zsmalloc.c | 3 +--
>>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>>>> index 1476289b619f..0b4b23740d78 100644
>>>> --- a/mm/zsmalloc.c
>>>> +++ b/mm/zsmalloc.c
>>>> @@ -1828,13 +1828,12 @@ static void putback_zspage_deferred(struct zs_pool *pool,
>>>>  static inline void zs_pool_dec_isolated(struct zs_pool *pool)
>>>>  {
>>>>         VM_BUG_ON(atomic_long_read(&pool->isolated_pages) <= 0);
>>>> -       atomic_long_dec(&pool->isolated_pages);
>>>>         /*
>>>>          * There's no possibility of racing, since wait_for_isolated_drain()
>>>>          * checks the isolated count under &class->lock after enqueuing
>>>>          * on migration_wait.
>>>>          */
>>>> -       if (atomic_long_read(&pool->isolated_pages) == 0 && pool->destroying)
>>>> +       if (atomic_long_dec_and_test(&pool->isolated_pages) && pool->destroying)
>>>>                 wake_up_all(&pool->migration_wait);
>>>>  }
>>>>
>>>> --
>>>> 2.23.0
>>>>
>>> .
>>>
>>
> .
>