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.
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
>
