[PATCH] block: fix race between switching elevator and removing queues

[PATCH] block: fix race between switching elevator and removing queues

[Ming Lei]

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@infradead.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-sysfs.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 107513495220..3af79831e717 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -1030,7 +1030,6 @@ 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;
@@ -1046,7 +1045,6 @@ void blk_unregister_queue(struct gendisk *disk)
 	 */
 	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);
@@ -1061,7 +1059,11 @@ void blk_unregister_queue(struct gendisk *disk)
 	kobject_del(&q->kobj);
 	blk_trace_remove_sysfs(disk_to_dev(disk));
 
-	if (has_elevator)
+	/*
+	 * q->kobj has been removed, so it is safe to check if elevator
+	 * exists without holding q->sysfs_lock.
+	 */
+	if (q->elevator)
 		elv_unregister_queue(q);
 	mutex_unlock(&q->sysfs_dir_lock);
 
-- 
2.20.1

Re: [PATCH] block: fix race between switching elevator and removing queues

[Jens Axboe]

On 9/11/19 10:02 PM, Ming Lei wrote:
> 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.

Thanks Ming, looks good to me and cleaner than before too.

-- 
Jens Axboe