[PATCH v3 2/3] writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()

[Baokun Li <libaokun@linux.alibaba.com>]

Commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when
switching inodes") replaced the queue_rcu_work() based scheduling of
inode wb switches with a plain queue_work().  Since then no switcher
goes through call_rcu(), so rcu_barrier() in cgroup_writeback_umount()
has no callbacks of its own to wait for.  It still drains unrelated
call_rcu() callbacks from other subsystems on busy systems, which
incidentally slows umount down; drop it.

Fixes: e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/fs-writeback.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 6766de9f9d75..325a30cc35bf 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1248,11 +1248,6 @@ void cgroup_writeback_umount(struct super_block *sb)
 		 * will then drain it.
 		 */
 		synchronize_rcu();
-		/*
-		 * Use rcu_barrier() to wait for all pending callbacks to
-		 * ensure that all in-flight wb switches are in the workqueue.
-		 */
-		rcu_barrier();
 		flush_workqueue(isw_wq);
 	}
 }
-- 
2.43.7