To obtain the pointer to the next memcg position, mem_cgroup_iter()
currently holds css->refcnt during memcg traversal only to put
css->refcnt at the end of the routine. This isn't necessary as an
rcu_read_lock is already held throughout the function. The use of
the RCU read lock with css_next_descendant_pre() guarantees that
sibling linkage is safe without holding a ref on the passed-in @css.
Remove css->refcnt usage during traversal by leveraging RCU.
Signed-off-by: Kinsey Ho <kinseyho@xxxxxxxxxx>
Reviewed-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Michal Koutný <mkoutny@xxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/memcontrol.c | 18 +-----------------
1 file changed, 1 insertion(+), 17 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dbc1d43a5c4c..cdd324812b55 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1015,20 +1015,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
else if (reclaim->generation != iter->generation)
goto out_unlock;
- while (1) {
- pos = READ_ONCE(iter->position);
- if (!pos || css_tryget(&pos->css))
- break;
- /*
- * css reference reached zero, so iter->position will
- * be cleared by ->css_released. However, we should not
- * rely on this happening soon, because ->css_released
- * is called from a work queue, and by busy-waiting we
- * might block it. So we clear iter->position right
- * away.
- */
- (void)cmpxchg(&iter->position, pos, NULL);
- }
+ pos = READ_ONCE(iter->position);
} else if (prev) {
pos = prev;
}
@@ -1069,9 +1056,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
*/
(void)cmpxchg(&iter->position, pos, memcg);
- if (pos)
- css_put(&pos->css);
-
if (!memcg)
iter->generation++;
}
--
2.46.0.469.g59c65b2a67-goog
