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@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.
>
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@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
>>>>>
>>>> .
>>>>
>>>
>> .
>>
>
