From b52602c6b36f687afa37076dca4ae61b264e31e3 Mon Sep 17 00:00:00 2001
Date: Sat, 11 Mar 2023 06:32:06 -0500
Subject: [PATCH] blk-cgroup: Revert changes causing NULL derefs on unmount

This reverts

- commit bfe46d2efe46 (blk-cgroup: synchronize pd_free_fn() from
  blkg_free_workfn() and blkcg_deactivate_policy(), 2023-01-19) and
- commit 57a425badc05 (blk-cgroup: dropping parent refcount after
  pd_free_fn() is done, 2023-01-19).

For: https://bugs.archlinux.org/task/77811
---
 block/blk-cgroup.c     | 39 ++++++++-------------------------------
 include/linux/blkdev.h |  1 -
 2 files changed, 8 insertions(+), 32 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index f8b21be..7c91d91 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -87,32 +87,14 @@ static void blkg_free_workfn(struct work_struct *work)
 {
 	struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
 					     free_work);
-	struct request_queue *q = blkg->q;
 	int i;
 
-	/*
-	 * pd_free_fn() can also be called from blkcg_deactivate_policy(),
-	 * in order to make sure pd_free_fn() is called in order, the deletion
-	 * of the list blkg->q_node is delayed to here from blkg_destroy(), and
-	 * blkcg_mutex is used to synchronize blkg_free_workfn() and
-	 * blkcg_deactivate_policy().
-	 */
-	if (q)
-		mutex_lock(&q->blkcg_mutex);
-
 	for (i = 0; i < BLKCG_MAX_POLS; i++)
 		if (blkg->pd[i])
 			blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
 
-	if (blkg->parent)
-		blkg_put(blkg->parent);
-
-	if (q) {
-		list_del_init(&blkg->q_node);
-		mutex_unlock(&q->blkcg_mutex);
-		blk_put_queue(q);
-	}
-
+	if (blkg->q)
+		blk_put_queue(blkg->q);
 	free_percpu(blkg->iostat_cpu);
 	percpu_ref_exit(&blkg->refcnt);
 	kfree(blkg);
@@ -145,6 +127,8 @@ static void __blkg_release(struct rcu_head *rcu)
 
 	/* release the blkcg and parent blkg refs this blkg has been holding */
 	css_put(&blkg->blkcg->css);
+	if (blkg->parent)
+		blkg_put(blkg->parent);
 	blkg_free(blkg);
 }
 
@@ -441,14 +425,9 @@ static void blkg_destroy(struct blkcg_gq *blkg)
 	lockdep_assert_held(&blkg->q->queue_lock);
 	lockdep_assert_held(&blkcg->lock);
 
-	/*
-	 * blkg stays on the queue list until blkg_free_workfn(), see details in
-	 * blkg_free_workfn(), hence this function can be called from
-	 * blkcg_destroy_blkgs() first and again from blkg_destroy_all() before
-	 * blkg_free_workfn().
-	 */
-	if (hlist_unhashed(&blkg->blkcg_node))
-		return;
+	/* Something wrong if we are trying to remove same group twice */
+	WARN_ON_ONCE(list_empty(&blkg->q_node));
+	WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));
 
 	for (i = 0; i < BLKCG_MAX_POLS; i++) {
 		struct blkcg_policy *pol = blkcg_policy[i];
@@ -460,6 +439,7 @@ static void blkg_destroy(struct blkcg_gq *blkg)
 	blkg->online = false;
 
 	radix_tree_delete(&blkcg->blkg_tree, blkg->q->id);
+	list_del_init(&blkg->q_node);
 	hlist_del_init_rcu(&blkg->blkcg_node);
 
 	/*
@@ -1246,7 +1226,6 @@ int blkcg_init_disk(struct gendisk *disk)
 	int ret;
 
 	INIT_LIST_HEAD(&q->blkg_list);
-	mutex_init(&q->blkcg_mutex);
 
 	new_blkg = blkg_alloc(&blkcg_root, disk, GFP_KERNEL);
 	if (!new_blkg)
@@ -1484,7 +1463,6 @@ void blkcg_deactivate_policy(struct request_queue *q,
 	if (queue_is_mq(q))
 		blk_mq_freeze_queue(q);
 
-	mutex_lock(&q->blkcg_mutex);
 	spin_lock_irq(&q->queue_lock);
 
 	__clear_bit(pol->plid, q->blkcg_pols);
@@ -1503,7 +1481,6 @@ void blkcg_deactivate_policy(struct request_queue *q,
 	}
 
 	spin_unlock_irq(&q->queue_lock);
-	mutex_unlock(&q->blkcg_mutex);
 
 	if (queue_is_mq(q))
 		blk_mq_unfreeze_queue(q);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 1680b6e..891f8cb 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -487,7 +487,6 @@ struct request_queue {
 	DECLARE_BITMAP		(blkcg_pols, BLKCG_MAX_POLS);
 	struct blkcg_gq		*root_blkg;
 	struct list_head	blkg_list;
-	struct mutex		blkcg_mutex;
 #endif
 
 	struct queue_limits	limits;
-- 
2.39.2