[RFC PATCH v2 1/4] blk-cgroup: wait for old blkgs to leave queue before disk rebind

[Yu Kuai <yukuai@kernel.org>]

From: Yu Kuai <yukuai@fygo.io>

blkcg_init_disk() currently waits for q->root_blkg to become NULL before
initializing blkcg state for a rebound disk. However, blkg_destroy_all()
clears q->root_blkg after calling blkg_destroy() for each blkg. At that
point the initial references have only been killed, and the blkgs remain
on q->blkg_list until the remaining references drain and
blkg_free_workfn() removes them.

A rebound disk can therefore install new blkcg state while old blkgs are
still attached to the request queue. Wait for q->blkg_list to become empty
instead, and wake the waiter when the final blkg is removed. This covers
the complete queue-side blkg lifetime without adding separate state.

Fixes: 3dbaacf6ab68 ("blk-cgroup: wait for blkcg cleanup before initializing new disk")
Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
 block/blk-cgroup.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 354637f3b158..229348273437 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -131,10 +131,12 @@ static void blkg_free_workfn(struct work_struct *work)
 			blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
 	if (blkg->parent)
 		blkg_put(blkg->parent);
 	spin_lock_irq(&q->queue_lock);
 	list_del_init(&blkg->q_node);
+	if (list_empty(&q->blkg_list))
+		wake_up_var(&q->blkg_list);
 	spin_unlock_irq(&q->queue_lock);
 	mutex_unlock(&q->blkcg_mutex);
 
 	/*
 	 * Release blkcg css ref only after blkg is removed from q->blkg_list,
@@ -607,12 +609,10 @@ static void blkg_destroy_all(struct gendisk *disk)
 	}
 
 	q->root_blkg = NULL;
 	spin_unlock_irq(&q->queue_lock);
 	mutex_unlock(&q->blkcg_mutex);
-
-	wake_up_var(&q->root_blkg);
 }
 
 static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
 {
 	int i;
@@ -1457,18 +1457,14 @@ int blkcg_init_disk(struct gendisk *disk)
 	bool preloaded;
 
 	/*
 	 * If the queue is shared across disk rebind (e.g., SCSI), the
 	 * previous disk's blkcg state is cleaned up asynchronously via
-	 * disk_release() -> blkcg_exit_disk(). Wait for that cleanup to
-	 * finish (indicated by root_blkg becoming NULL) before setting up
-	 * new blkcg state. Otherwise, we may overwrite q->root_blkg while
-	 * the old one is still alive, and radix_tree_insert() in
-	 * blkg_create() will fail with -EEXIST because the old entries
-	 * still occupy the same queue id slot in blkcg->blkg_tree.
+	 * disk_release() -> blkcg_exit_disk(). Wait for all old blkgs to be
+	 * removed from the queue list before setting up new blkcg state.
 	 */
-	wait_var_event(&q->root_blkg, !READ_ONCE(q->root_blkg));
+	wait_var_event(&q->blkg_list, list_empty_careful(&q->blkg_list));
 
 	new_blkg = blkg_alloc(&blkcg_root, disk, GFP_KERNEL);
 	if (!new_blkg)
 		return -ENOMEM;
 
-- 
2.51.0