[PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

[Waiman Long <longman@redhat.com>]

Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"),
blkg's stats is only flushed if they are online. In addition, the
stat flushing of blkgs in blkcg_rstat_flush() includes propagating
the rstat data to its parent. However, if a blkg has been destroyed
(offline), the validity of its parent may be questionable. For safety,
revert back to the old behavior by ignoring offline blkg's.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 block/blk-cgroup.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 21cc88349f21..c466aef0d467 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -885,6 +885,12 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
 
 		WRITE_ONCE(bisc->lqueued, false);
 
+		/* Don't flush its stats if blkg is offline */
+		if (unlikely(!blkg->online)) {
+			percpu_ref_put(&blkg->refcnt);
+			continue;
+		}
+
 		/* fetch the current per-cpu values */
 		do {
 			seq = u64_stats_fetch_begin(&bisc->sync);
-- 
2.31.1