On Fri 02-08-19 12:22:41, Roman Gushchin wrote:
> Commit 72f0184c8a00 ("mm, memcg: remove hotplug locking from try_charge")
> introduced css_tryget()/css_put() calls in drain_all_stock(),
> which are supposed to protect the target memory cgroup from being
> released during the mem_cgroup_is_descendant() call.
>
> However, it's not completely safe. In theory, memcg can go away
> between reading stock->cached pointer and calling css_tryget().
>
> This can happen if drain_all_stock() races with drain_local_stock()
> performed on the remote cpu as a result of a work, scheduled
> by the previous invocation of drain_all_stock().
Maybe I am still missing something but I do not see how 72f0184c8a00
changed the existing race. get_online_cpus doesn't prevent the same race
right? If this is the case then it would be great to clarify that. I
know that you are mostly after clarifying that css_tryget is
insufficient but the above sounds like 72f0184c8a00 has introduced a
regression.
> The race is a bit theoretical and there are few chances to trigger
> it, but the current code looks a bit confusing, so it makes sense
> to fix it anyway. The code looks like as if css_tryget() and
> css_put() are used to protect stocks drainage. It's not necessary
> because stocked pages are holding references to the cached cgroup.
> And it obviously won't work for works, scheduled on other cpus.
>
> So, let's read the stock->cached pointer and evaluate the memory
> cgroup inside a rcu read section, and get rid of
> css_tryget()/css_put() calls.
>
> v2: added some explanations to the commit message, no code changes
>
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Hillf Danton <hdanton@xxxxxxxx>
Other than that.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/memcontrol.c | 17 +++++++++--------
> 1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5c7b9facb0eb..d856b64426b7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2235,21 +2235,22 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> for_each_online_cpu(cpu) {
> struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> struct mem_cgroup *memcg;
> + bool flush = false;
>
> + rcu_read_lock();
> memcg = stock->cached;
> - if (!memcg || !stock->nr_pages || !css_tryget(&memcg->css))
> - continue;
> - if (!mem_cgroup_is_descendant(memcg, root_memcg)) {
> - css_put(&memcg->css);
> - continue;
> - }
> - if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> + if (memcg && stock->nr_pages &&
> + mem_cgroup_is_descendant(memcg, root_memcg))
> + flush = true;
> + rcu_read_unlock();
> +
> + if (flush &&
> + !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> if (cpu == curcpu)
> drain_local_stock(&stock->work);
> else
> schedule_work_on(cpu, &stock->work);
> }
> - css_put(&memcg->css);
> }
> put_cpu();
> mutex_unlock(&percpu_charge_mutex);
> --
> 2.21.0
--
Michal Hocko
SUSE Labs
