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@xxxxxxxxxxxxx> Cc: Hannes Reinecke <hare@xxxxxxxx> Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: Mike Snitzer <snitzer@xxxxxxxxxx> Cc: Bart Van Assche <bvanassche@xxxxxxx> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> --- 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
