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    Subject[PATCH 4.4 66/86] blk-mq: Allow blocking queue tag iter callbacks
    Date
    From: Keith Busch <keith.busch@intel.com>

    commit 530ca2c9bd6949c72c9b5cfc330cb3dbccaa3f5b upstream.

    A recent commit runs tag iterator callbacks under the rcu read lock,
    but existing callbacks do not satisfy the non-blocking requirement.
    The commit intended to prevent an iterator from accessing a queue that's
    being modified. This patch fixes the original issue by taking a queue
    reference instead of reading it, which allows callbacks to make blocking
    calls.

    Fixes: f5bbbbe4d6357 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
    Acked-by: Jianchao Wang <jianchao.w.wang@oracle.com>
    Signed-off-by: Keith Busch <keith.busch@intel.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Giuliano Procida <gprocida@google.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    block/blk-mq-tag.c | 7 ++-----
    1 file changed, 2 insertions(+), 5 deletions(-)

    --- a/block/blk-mq-tag.c
    +++ b/block/blk-mq-tag.c
    @@ -484,11 +484,8 @@ void blk_mq_queue_tag_busy_iter(struct r
    /*
    * Avoid potential races with things like queue removal.
    */
    - rcu_read_lock();
    - if (percpu_ref_is_zero(&q->q_usage_counter)) {
    - rcu_read_unlock();
    + if (!percpu_ref_tryget(&q->q_usage_counter))
    return;
    - }

    queue_for_each_hw_ctx(q, hctx, i) {
    struct blk_mq_tags *tags = hctx->tags;
    @@ -505,7 +502,7 @@ void blk_mq_queue_tag_busy_iter(struct r
    bt_for_each(hctx, &tags->bitmap_tags, tags->nr_reserved_tags, fn, priv,
    false);
    }
    - rcu_read_unlock();
    + blk_queue_exit(q);
    }

    static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)

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