[PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock()

[Roman Gushchin]

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().

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.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: 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