[PATCH v4 4/7] blk-mq: fix q->mq_usage_counter access race

From: Akinobu Mita
Date: Sat Sep 26 2015 - 13:10:11 EST

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@xxxxxxxxx>
Reviewed-by: Ming Lei <tom.leiming@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Ming Lei <tom.leiming@xxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
block/blk-mq.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 92648d8..3a39184 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_exit_hw_queues(q, set, set->nr_hw_queues);
blk_mq_free_hw_queues(q, set);

- 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 */

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