[PATCH mm-unstable v4 2/5] mm: don't hold css->refcnt during traversal

[Kinsey Ho <kinseyho@google.com>]

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@google.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Hugh Dickins <hughd@google.com>
---
 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