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

David Jeffery
2017-08-07T19:38:23Z

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
*/