Commit c48dac13 authored Aug 27, 2019 by Ming Lei Committed by Jens Axboe Aug 27, 2019

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>

parent 9685b227

block/elevator.c

+5 −9

Original line number	Diff line number	Diff line
		@@ -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;
		}

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