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Commit b89f625e authored by Ming Lei's avatar Ming Lei Committed by Jens Axboe
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block: don't release queue's sysfs lock during switching elevator 



cecf5d87 ("block: split .sysfs_lock into two locks") starts to
release & acquire sysfs_lock before registering/un-registering elevator
queue during switching elevator for avoiding potential deadlock from
showing & storing 'queue/iosched' attributes and removing elevator's
kobject.

Turns out there isn't such deadlock because 'q->sysfs_lock' isn't
required in .show & .store of queue/iosched's attributes, and just
elevator's sysfs lock is acquired in elv_iosched_store() and
elv_iosched_show(). So it is safe to hold queue's sysfs lock when
registering/un-registering elevator queue.

The biggest issue is that commit cecf5d87 assumes that concurrent
write on 'queue/scheduler' can't happen. However, this assumption isn't
true, because kernfs_fop_write() only guarantees that concurrent write
aren't called on the same open file, but the write could be from
different open on the file. So we can't release & re-acquire queue's
sysfs lock during switching elevator, otherwise use-after-free on
elevator could be triggered.

Fixes the issue by not releasing queue's sysfs lock during switching
elevator.

Fixes: cecf5d87 ("block: split .sysfs_lock into two locks")
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: default avatarBart Van Assche <bvanassche@acm.org>
Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 284b94be

block/blk-sysfs.c

+4 −9
Original line number Diff line number Diff line
@@ -989,13 +989,11 @@ int blk_register_queue(struct gendisk *disk) 
		blk_mq_debugfs_register(q);

	}



	/*

	 * The flag of QUEUE_FLAG_REGISTERED isn't set yet, so elevator

	 * switch won't happen at all.

	 */

	mutex_lock(&q->sysfs_lock);

	if (q->elevator) {

		ret = elv_register_queue(q, false);

		if (ret) {

			mutex_unlock(&q->sysfs_lock);

			mutex_unlock(&q->sysfs_dir_lock);

			kobject_del(&q->kobj);

			blk_trace_remove_sysfs(dev);
@@ -1005,7 +1003,6 @@ int blk_register_queue(struct gendisk *disk) 
		has_elevator = true;

	}



	mutex_lock(&q->sysfs_lock);

	blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);

	wbt_enable_default(q);

	blk_throtl_register_queue(q);
@@ -1062,12 +1059,10 @@ void blk_unregister_queue(struct gendisk *disk) 
	kobject_del(&q->kobj);

	blk_trace_remove_sysfs(disk_to_dev(disk));



	/*

	 * q->kobj has been removed, so it is safe to check if elevator

	 * exists without holding q->sysfs_lock.

	 */

	mutex_lock(&q->sysfs_lock);

	if (q->elevator)

		elv_unregister_queue(q);

	mutex_unlock(&q->sysfs_lock);

	mutex_unlock(&q->sysfs_dir_lock);



	kobject_put(&disk_to_dev(disk)->kobj);

block/elevator.c

+1 −30
Original line number Diff line number Diff line
@@ -503,9 +503,7 @@ int elv_register_queue(struct request_queue *q, bool uevent) 
		if (uevent)

			kobject_uevent(&e->kobj, KOBJ_ADD);



		mutex_lock(&q->sysfs_lock);

		e->registered = 1;

		mutex_unlock(&q->sysfs_lock);

	}

	return error;

}
@@ -523,11 +521,9 @@ void elv_unregister_queue(struct request_queue *q) 
		kobject_uevent(&e->kobj, KOBJ_REMOVE);

		kobject_del(&e->kobj);



		mutex_lock(&q->sysfs_lock);

		e->registered = 0;

		/* Re-enable throttling in case elevator disabled it */

		wbt_enable_default(q);

		mutex_unlock(&q->sysfs_lock);

	}

}
@@ -590,32 +586,11 @@ int elevator_switch_mq(struct request_queue *q, 
	lockdep_assert_held(&q->sysfs_lock);



	if (q->elevator) {

		if (q->elevator->registered) {

			mutex_unlock(&q->sysfs_lock);



			/*

			 * Concurrent elevator switch can't happen becasue

			 * sysfs write is always exclusively on same file.

			 *

			 * Also the elevator queue won't be freed after

			 * sysfs_lock is released becasue kobject_del() in

			 * blk_unregister_queue() waits for completion of

			 * .store & .show on its attributes.

			 */

		if (q->elevator->registered)

			elv_unregister_queue(q);



			mutex_lock(&q->sysfs_lock);

		}

		ioc_clear_queue(q);

		elevator_exit(q, q->elevator);



		/*

		 * sysfs_lock may be dropped, so re-check if queue is

		 * unregistered. If yes, don't switch to new elevator

		 * any more

		 */

		if (!blk_queue_registered(q))

			return 0;

	}



	ret = blk_mq_init_sched(q, new_e);
@@ -623,11 +598,7 @@ int elevator_switch_mq(struct request_queue *q, 
		goto out;



	if (new_e) {

		mutex_unlock(&q->sysfs_lock);



		ret = elv_register_queue(q, true);



		mutex_lock(&q->sysfs_lock);

		if (ret) {

			elevator_exit(q, q->elevator);

			goto out;