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From:   Waiman Long <longman@redhat.com>
To:     Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        =?UTF-8?q?Michal=20Koutn=C3=BD?= <mkoutny@suse.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Waiman Long <longman@redhat.com>
Subject: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed
Date:   Sun, 11 Dec 2022 17:20:57 -0500
Message-Id: <20221211222058.2946830-3-longman@redhat.com>
In-Reply-To: <20221211222058.2946830-1-longman@redhat.com>
References: <20221211222058.2946830-1-longman@redhat.com>
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Precedence: bulk

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