[GIT PULL] single block fix for 2.6.36

[Jens Axboe <jaxboe@fusionio.com>]

Hi Linus,

The API that was added for drivers to switch IO schedulers
when loaded does not work if the driver isn't in a fully
initialized state. The in-kernel ones call it right after
blk_init_queue(), which will result in an oops when the
elevator core tries to unregister unregistered kobjects.

Add a registered bit and only do the unregister/register
dance in elevator_switch() if we need to. The other call
path for this is the sysfs parts to allow online switching,
which can only be called with a fully setup driver.

Please pull, this is a regression introduced in this series.

are available in the git repository at:
  git://git.kernel.dk/linux-2.6-block.git for-linus

Jens Axboe (1):
      elevator: fix oops on early call to elevator_change()

 block/elevator.c         |   12 ++++++++----
 include/linux/elevator.h |    1 +
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 205b09a..4e11559 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -938,6 +938,7 @@ int elv_register_queue(struct request_queue *q)
 			}
 		}
 		kobject_uevent(&e->kobj, KOBJ_ADD);
+		e->registered = 1;
 	}
 	return error;
 }
@@ -947,6 +948,7 @@ static void __elv_unregister_queue(struct elevator_queue *e)
 {
 	kobject_uevent(&e->kobj, KOBJ_REMOVE);
 	kobject_del(&e->kobj);
+	e->registered = 0;
 }
 
 void elv_unregister_queue(struct request_queue *q)
@@ -1042,11 +1044,13 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 
 	spin_unlock_irq(q->queue_lock);
 
-	__elv_unregister_queue(old_elevator);
+	if (old_elevator->registered) {
+		__elv_unregister_queue(old_elevator);
 
-	err = elv_register_queue(q);
-	if (err)
-		goto fail_register;
+		err = elv_register_queue(q);
+		if (err)
+			goto fail_register;
+	}
 
 	/*
 	 * finally exit old elevator and turn off BYPASS.
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 926b503..4fd978e 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -93,6 +93,7 @@ struct elevator_queue
 	struct elevator_type *elevator_type;
 	struct mutex sysfs_lock;
 	struct hlist_head *hash;
+	unsigned int registered:1;
 };
 
 /*

-- 
Jens Axboe