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

[Muchun Song]

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.

>
> > 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
> >>
> > .
> >
>