[PATCH v5 6/8] mm: vmscan: hold write lock to reparent shrinker nr_deferred

[Qi Zheng <zhengqi.arch@bytedance.com>]

For now, reparent_shrinker_deferred() is the only holder
of read lock of shrinker_rwsem. And it already holds the
global cgroup_mutex, so it will not be called in parallel.

Therefore, in order to convert shrinker_rwsem to shrinker_mutex
later, here we change to hold the write lock of shrinker_rwsem
to reparent.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill Tkhai <tkhai@ya.ru>
---
 mm/vmscan.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5c2a22454320..8c1ae7ea8dea 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -445,7 +445,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
 		parent = root_mem_cgroup;
 
 	/* Prevent from concurrent shrinker_info expand */
-	down_read(&shrinker_rwsem);
+	down_write(&shrinker_rwsem);
 	for_each_node(nid) {
 		child_info = shrinker_info_protected(memcg, nid);
 		parent_info = shrinker_info_protected(parent, nid);
@@ -454,7 +454,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
 			atomic_long_add(nr, &parent_info->nr_deferred[i]);
 		}
 	}
-	up_read(&shrinker_rwsem);
+	up_write(&shrinker_rwsem);
 }
 
 static bool cgroup_reclaim(struct scan_control *sc)
-- 
2.20.1