On 8/21/19 2:15 AM, Ming Lei wrote:
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
^^^^^^^^^^^^
only?
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().
[ ... ]
/*
- * 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.
*/
How about replacing this comment with the following:
WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));
Anyway:
Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>
