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>
---
include/linux/memcontrol.h | 2 +-
mm/memcontrol.c | 18 +-----------------
2 files changed, 2 insertions(+), 18 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 90ecd2dbca06..1aaed2f1f6ae 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -75,7 +75,7 @@ struct lruvec_stats_percpu;
struct lruvec_stats;
struct mem_cgroup_reclaim_iter {
- struct mem_cgroup *position;
+ struct mem_cgroup __rcu *position;
/* scan generation, increased every round-trip */
unsigned int generation;
};
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dacf4fec4541..1688aae3b1b4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1052,20 +1052,7 @@ 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 = rcu_dereference(iter->position);
} else if (prev) {
pos = prev;
}
@@ -1106,9 +1093,6 @@ 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++;
}
--
2.46.0.76.ge559c4bf1a-goog
