blk-mq: fix q->mq_usage_counter access race

CPU hotplug handling for blk-mq (blk_mq_queue_reinit) accesses
q->mq_usage_counter while freezing all request queues in all_q_list.
On the other hand, q->mq_usage_counter is deinitialized in
blk_mq_free_queue() before deleting the queue from all_q_list.

So if CPU hotplug event occurs in the window, percpu_ref_kill() is
called with q->mq_usage_counter which has already been marked dead,
and it triggers warning.  Fix it by deleting the queue from all_q_list
earlier than destroying q->mq_usage_counter.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Cc: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 92648d8d6a4a..3a39184e82e5 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2067,15 +2067,16 @@ void blk_mq_free_queue(struct request_queue *q)
 {
         struct blk_mq_tag_set   *set = q->tag_set;
 
+        mutex_lock(&all_q_mutex);
+        list_del_init(&q->all_q_node);
+        mutex_unlock(&all_q_mutex);
+
         blk_mq_del_queue_tag_set(q);
 
         blk_mq_exit_hw_queues(q, set, set->nr_hw_queues);
         blk_mq_free_hw_queues(q, set);
 
         percpu_ref_exit(&q->mq_usage_counter);
-        mutex_lock(&all_q_mutex);
-        list_del_init(&q->all_q_node);
-        mutex_unlock(&all_q_mutex);
 }
 
 /* Basically redo blk_mq_init_queue with queue frozen */