block: don't hold q->sysfs_lock in elevator_init_mq

The original comment says:

	q->sysfs_lock must be held to provide mutual exclusion between
	elevator_switch() and here.

Which is simply wrong. elevator_init_mq() is only called from
blk_mq_init_allocated_queue, which is always called before the request
queue is registered via blk_register_queue(), for dm-rq or normal rq
based driver. However, queue's kobject is only exposed and added to sysfs
in blk_register_queue(). So there isn't such race between elevator_switch()
and elevator_init_mq().

So avoid to hold q->sysfs_lock in elevator_init_mq().

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 5 insertions, 9 deletions

diff --git a/block/elevator.c b/block/elevator.c
index 2f17d66d0e61..33c15fb54ed1 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -607,23 +607,19 @@ int elevator_init_mq(struct request_queue *q)
         if (q->nr_hw_queues != 1)
                 return 0;
 
-        /*
-         * q->sysfs_lock must be held to provide mutual exclusion between
-         * elevator_switch() and here.
-         */
-        mutex_lock(&q->sysfs_lock);
+        WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));
+
         if (unlikely(q->elevator))
-                goto out_unlock;
+                goto out;
 
         e = elevator_get(q, "mq-deadline", false);
         if (!e)
-                goto out_unlock;
+                goto out;
 
         err = blk_mq_init_sched(q, e);
         if (err)
                 elevator_put(e);
-out_unlock:
-        mutex_unlock(&q->sysfs_lock);
+out:
         return err;
 }