[PATCH V2 2/6] block: don't hold q->sysfs_lock in elevator_init_mq

[Ming Lei <ming.lei@redhat.com>]

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 just exposed 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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/elevator.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 2f17d66d0e61..37b918dc4676 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -608,22 +608,22 @@ int elevator_init_mq(struct request_queue *q)
 		return 0;
 
 	/*
-	 * q->sysfs_lock must be held to provide mutual exclusion between
-	 * elevator_switch() and here.
+	 * We are called from blk_mq_init_allocated_queue() only, at that
+	 * time the request queue isn't registered yet, so the queue
+	 * kobject isn't exposed to userspace. No need to worry about race
+	 * with elevator_switch(), and no need to hold q->sysfs_lock.
 	 */
-	mutex_lock(&q->sysfs_lock);
 	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;
 }
 
-- 
2.20.1