[PATCH] block: Fix warning when I/O elevator is changed as request_queue is being removed

[David Jeffery <djeffery@redhat.com>]

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>
---
 block/blk-sysfs.c |    2 ++
 block/elevator.c  |    4 ++++
 2 files changed, 6 insertions(+)


diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 27aceab..b8362c0 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -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);
 
diff --git a/block/elevator.c b/block/elevator.c
index 4bb2f0c..51da592 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -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
 	 */