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    Subject[PATCH v4 4/7] blk-mq: fix q->mq_usage_counter access race
    Date
    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: Jens Axboe <axboe@kernel.dk>
    Cc: Ming Lei <tom.leiming@gmail.com>
    Cc: Christoph Hellwig <hch@lst.de>
    ---
    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_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 */
    --
    1.9.1


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