linux-kernel - [PATCH 4.19 20/24] block: dont hold q->sysfs_lock in elevator_init

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Message-Id: <20210211150148.637345944@linuxfoundation.org>
Date:   Thu, 11 Feb 2021 16:02:54 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Hannes Reinecke <hare@...e.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Bart Van Assche <bvanassche@....org>,
        Damien Le Moal <damien.lemoal@....com>,
        Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Jack Wang <jinpu.wang@...ud.ionos.com>
Subject: [PATCH 4.19 20/24] block: dont hold q->sysfs_lock in elevator_init_mq

From: Ming Lei <ming.lei@...hat.com>

commit c48dac137a62a5d6fa1ef3fa445cbd9c43655a76 upstream.

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@...radead.org>
Cc: Hannes Reinecke <hare@...e.com>
Cc: Greg KH <gregkh@...uxfoundation.org>
Cc: Mike Snitzer <snitzer@...hat.com>
Cc: Bart Van Assche <bvanassche@....org>
Cc: Damien Le Moal <damien.lemoal@....com>
Reviewed-by: Bart Van Assche <bvanassche@....org>
Signed-off-by: Ming Lei <ming.lei@...hat.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Jack Wang <jinpu.wang@...ud.ionos.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 block/elevator.c |   14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

--- a/block/elevator.c
+++ b/block/elevator.c
@@ -980,23 +980,19 @@ int elevator_init_mq(struct request_queu
 	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;
 }