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

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/6/25 16:46, Miaohe Lin wrote:
> On 2021/6/25 15:29, Muchun Song wrote:
>> On Fri, Jun 25, 2021 at 2:32 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
>>>
>>> On 2021/6/25 13:01, Muchun Song wrote:
>>>> On Thu, Jun 24, 2021 at 8:40 PM Miaohe Lin <linmiaohe@huawei.com> 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.
> 

It seems there is race between zs_pool_dec_isolated and zs_unregister_migration if pool->destroying
is reordered before the atomic_long_dec and atomic_long_read ops. So this memory barrier is necessary:

zs_pool_dec_isolated				zs_unregister_migration
  pool->destroying != true
						  pool->destroying = true;
						  smp_mb();
						  wait_for_isolated_drain
						    wait_event with atomic_long_read(&pool->isolated_pages) != 0
  atomic_long_dec(&pool->isolated_pages);
  atomic_long_read(&pool->isolated_pages) == 0

Thus wake_up_all is missed.
And the comment in zs_pool_dec_isolated() said:
/*
 * There's no possibility of racing, since wait_for_isolated_drain()
 * checks the isolated count under &class->lock after enqueuing
 * on migration_wait.
 */

But I found &class->lock is indeed not acquired for wait_for_isolated_drain(). So I think the above race
is possible. Does this make senses for you ?
Thanks.

>>
>>>
>>>> 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@huawei.com>
>>>>> ---
>>>>>  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
>>>>>
>>>> .
>>>>
>>>
>> .
>>
>