Commit e9a823fb authored Aug 28, 2017 by David Jeffery Committed by Jens Axboe Aug 28, 2017

block: fix warning when I/O elevator is changed as request_queue is being removed



There is a race between changing I/O elevator and request_queue removal
which can trigger the warning in kobject_add_internal.  A program can
use sysfs to request a change of elevator at the same time another task
is unregistering the request_queue the elevator would be attached to.
The elevator's kobject will then attempt to be connected to the
request_queue in the object tree when the request_queue has just been
removed from sysfs.  This triggers the warning in kobject_add_internal
as the request_queue no longer has a sysfs directory:

kobject_add_internal failed for iosched (error: -2 parent: queue)
------------[ cut here ]------------
WARNING: CPU: 3 PID: 14075 at lib/kobject.c:244 kobject_add_internal+0x103/0x2d0

To fix this warning, we can check the QUEUE_FLAG_REGISTERED flag when
changing the elevator and use the request_queue's sysfs_lock to
serialize between clearing the flag and the elevator testing the flag.

Signed-off-by: David Jeffery <djeffery@redhat.com>
Tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 235f8da1

block/blk-sysfs.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -931,7 +931,9 @@ void blk_unregister_queue(struct gendisk *disk)
		if (WARN_ON(!q))
		return;

		mutex_lock(&q->sysfs_lock);
		queue_flag_clear_unlocked(QUEUE_FLAG_REGISTERED, q);
		mutex_unlock(&q->sysfs_lock);

		wbt_exit(q);

block/elevator.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -1055,6 +1055,10 @@ static int __elevator_change(struct request_queue q, const char name)
		char elevator_name[ELV_NAME_MAX];
		struct elevator_type *e;

		/* Make sure queue is not in the middle of being removed */
		if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
		return -ENOENT;

		/*
		* Special case for mq, turn off scheduling
		*/

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