From: Tejun Heo <tj@kernel.org>
Date: Wed, 24 Jul 2019 10:37:55 -0700
Subject: [PATCH] blkcg: don't offline parent blkcg first
References: bsc#1175995,jsc#SLE-15608
Git-commit: 4308a434e5e08c78676aa66bc626ef78cbef0883
Patch-mainline: v5.7-rc1

blkcg->cgwb_refcnt is used to delay blkcg offlining so that blkgs
don't get offlined while there are active cgwbs on them.  However, it
ends up making offlining unordered sometimes causing parents to be
offlined before children.

Let's fix this by making child blkcgs pin the parents' online states.

Note that pin/unpin names are chosen over get/put intentionally
because css uses get/put online for something different.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Hannes Reinecke <hare@suse.de>
---
 block/blk-cgroup.c         | 16 ++++++++++++++++
 include/linux/blk-cgroup.h |  6 +++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 445997d76212..4664d6e40d0e 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1006,6 +1006,21 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 	return ret;
 }
 
+static int blkcg_css_online(struct cgroup_subsys_state *css)
+{
+	struct blkcg *blkcg = css_to_blkcg(css);
+	struct blkcg *parent = blkcg_parent(blkcg);
+
+	/*
+	 * blkcg_pin_online() is used to delay blkcg offline so that blkgs
+	 * don't go offline while cgwbs are still active on them.  Pin the
+	 * parent so that offline always happens towards the root.
+	 */
+	if (parent)
+		blkcg_pin_online(parent);
+	return 0;
+}
+
 /**
  * blkcg_init_queue - initialize blkcg part of request queue
  * @q: request_queue to initialize
@@ -1199,6 +1214,7 @@ static void blkcg_exit(struct task_struct *tsk)
 
 struct cgroup_subsys io_cgrp_subsys = {
 	.css_alloc = blkcg_css_alloc,
+	.css_online = blkcg_css_online,
 	.css_offline = blkcg_css_offline,
 	.css_free = blkcg_css_free,
 	.can_attach = blkcg_can_attach,
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 63172c90ca14..be71b166caae 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -436,8 +436,12 @@ static inline void blkcg_pin_online(struct blkcg *blkcg)
  */
 static inline void blkcg_unpin_online(struct blkcg *blkcg)
 {
-	if (refcount_dec_and_test(&blkcg->online_pin))
+	do {
+		if (!refcount_dec_and_test(&blkcg->online_pin))
+			break;
 		blkcg_destroy_blkgs(blkcg);
+		blkcg = blkcg_parent(blkcg);
+	} while (blkcg);
 }
 
 /**
-- 
2.16.4