blkcg: don't offline parent blkcg first

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>

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,

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);
 }
 
 /**