linux-kernel - Re: [Phishing Risk] [External] [PATCH 2/3] mm/zsmalloc.c: combine two atomic ops in zs_pool_dec

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Date:   Fri, 25 Jun 2021 13:01:53 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>, ngupta@...are.org,
        senozhatsky@...omium.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [Phishing Risk] [External] [PATCH 2/3] mm/zsmalloc.c: combine two
 atomic ops in zs_pool_dec_isolated()

On Thu, Jun 24, 2021 at 8:40 PM Miaohe Lin <linmiaohe@...wei.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.

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@...wei.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
>