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 <h...@infradead.org>
Cc: Hannes Reinecke <h...@suse.com>
Cc: Greg KH <gre...@linuxfoundation.org>
Cc: Mike Snitzer <snit...@redhat.com>
Cc: Bart Van Assche <bvanass...@acm.org>
Signed-off-by: Ming Lei <ming....@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
