I’d go with atomic_dec().
Sent from my iPhone
> On Aug 3, 2021, at 00:11, Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 03-08-21 14:29:13, Miaohe Lin wrote:
> [...]
>> 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
