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    Subject[PATCH 4.14 09/38] blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter
    Date
    From: Jianchao Wang <jianchao.w.wang@oracle.com>

    commit f5bbbbe4d63577026f908a809f22f5fd5a90ea1f upstream.

    For blk-mq, part_in_flight/rw will invoke blk_mq_in_flight/rw to
    account the inflight requests. It will access the queue_hw_ctx and
    nr_hw_queues w/o any protection. When updating nr_hw_queues and
    blk_mq_in_flight/rw occur concurrently, panic comes up.

    Before update nr_hw_queues, the q will be frozen. So we could use
    q_usage_counter to avoid the race. percpu_ref_is_zero is used here
    so that we will not miss any in-flight request. The access to
    nr_hw_queues and queue_hw_ctx in blk_mq_queue_tag_busy_iter are
    under rcu critical section, __blk_mq_update_nr_hw_queues could use
    synchronize_rcu to ensure the zeroed q_usage_counter to be globally
    visible.

    Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Cc: Giuliano Procida <gprocida@google.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    block/blk-mq-tag.c | 14 +++++++++++++-
    block/blk-mq.c | 4 ++++
    2 files changed, 17 insertions(+), 1 deletion(-)

    --- a/block/blk-mq-tag.c
    +++ b/block/blk-mq-tag.c
    @@ -334,6 +334,18 @@ void blk_mq_queue_tag_busy_iter(struct r
    struct blk_mq_hw_ctx *hctx;
    int i;

    + /*
    + * __blk_mq_update_nr_hw_queues will update the nr_hw_queues and
    + * queue_hw_ctx after freeze the queue. So we could use q_usage_counter
    + * to avoid race with it. __blk_mq_update_nr_hw_queues will users
    + * synchronize_rcu to ensure all of the users go out of the critical
    + * section below and see zeroed q_usage_counter.
    + */
    + rcu_read_lock();
    + if (percpu_ref_is_zero(&q->q_usage_counter)) {
    + rcu_read_unlock();
    + return;
    + }

    queue_for_each_hw_ctx(q, hctx, i) {
    struct blk_mq_tags *tags = hctx->tags;
    @@ -349,7 +361,7 @@ void blk_mq_queue_tag_busy_iter(struct r
    bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
    bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
    }
    -
    + rcu_read_unlock();
    }

    static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,
    --- a/block/blk-mq.c
    +++ b/block/blk-mq.c
    @@ -2748,6 +2748,10 @@ static void __blk_mq_update_nr_hw_queues

    list_for_each_entry(q, &set->tag_list, tag_set_list)
    blk_mq_unfreeze_queue(q);
    + /*
    + * Sync with blk_mq_queue_tag_busy_iter.
    + */
    + synchronize_rcu();
    }

    void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)

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