I’d go with atomic_dec().

Sent from my iPhone

On Aug 3, 2021, at 00:11, Michal Hocko [off-list ref] wrote:

On Tue 03-08-21 14:29:13, Miaohe Lin wrote:
[...]

quoted

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 616d1a72ece3..6210b1124929 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2208,11 +2208,11 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 */

static void drain_all_stock(struct mem_cgroup *root_memcg)
{
-       static DEFINE_MUTEX(percpu_charge_mutex);
       int cpu, curcpu;
+       static atomic_t drainer = ATOMIC_INIT(0);

       /* If someone's already draining, avoid adding running more workers. */
-       if (!mutex_trylock(&percpu_charge_mutex))
+       if (atomic_cmpxchg(&drainer, 0, 1) != 0)
               return;
       /*
        * Notify other cpus that system-wide "drain" is running

@@ -2244,7 +2244,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
               }
       }
       put_cpu();
-       mutex_unlock(&percpu_charge_mutex);
+       atomic_set(&drainer, 0);

atomic_set doesn't imply memory barrier IIRC. Is this safe?

-- 
Michal Hocko
SUSE Labs