[PATCH 4.19 010/247] block: fix race between switching elevator and removing queues

[Greg Kroah-Hartman]

From: Ming Lei <ming.lei@xxxxxxxxxx>

commit 0a67b5a926e63ff5492c3c675eab5900580d056d upstream.

cecf5d87ff20 ("block: split .sysfs_lock into two locks") starts to
release & actuire sysfs_lock again during switching elevator. So it
isn't enough to prevent switching elevator from happening by simply
clearing QUEUE_FLAG_REGISTERED with holding sysfs_lock, because
in-progress switch still can move on after re-acquiring the lock,
meantime the flag of QUEUE_FLAG_REGISTERED won't get checked.

Fixes this issue by checking 'q->elevator' directly & locklessly after
q->kobj is removed in blk_unregister_queue(), this way is safe because
q->elevator can't be changed at that time.

Fixes: cecf5d87ff20 ("block: split .sysfs_lock into two locks")
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: Bart Van Assche <bvanassche@xxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Jack Wang <jinpu.wang@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 block/blk-sysfs.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -977,7 +977,6 @@ EXPORT_SYMBOL_GPL(blk_register_queue);
 void blk_unregister_queue(struct gendisk *disk)
 {
 	struct request_queue *q = disk->queue;
-	bool has_elevator;
 
 	if (WARN_ON(!q))
 		return;
@@ -993,7 +992,6 @@ void blk_unregister_queue(struct gendisk
 	 */
 	mutex_lock(&q->sysfs_lock);
 	blk_queue_flag_clear(QUEUE_FLAG_REGISTERED, q);
-	has_elevator = !!q->elevator;
 	mutex_unlock(&q->sysfs_lock);
 
 	mutex_lock(&q->sysfs_dir_lock);
@@ -1009,7 +1007,11 @@ void blk_unregister_queue(struct gendisk
 	blk_trace_remove_sysfs(disk_to_dev(disk));
 
 	mutex_lock(&q->sysfs_lock);
-	if (q->request_fn || has_elevator)
+	/*
+	 * q->kobj has been removed, so it is safe to check if elevator
+	 * exists without holding q->sysfs_lock.
+	 */
+	if (q->request_fn || q->elevator)
 		elv_unregister_queue(q);
 	mutex_unlock(&q->sysfs_lock);
 	mutex_unlock(&q->sysfs_dir_lock);