From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 7 Oct 2010 20:54:56 +0000 (-0700)
Subject: Merge branch 'for-linus' of git://git.kernel.dk/linux-2.6-block
X-Git-Tag: v2.6.36-rc8~27
X-Git-Url: http://bbs.cooldavid.org/git/?a=commitdiff_plain;h=c8d86d8ac439ef6c9364bf0ba3a05a3f9a8e0dea;hp=74de82ed1e49d9c7af84c3beb3759316854f54ed;p=net-next-2.6.git

Merge branch 'for-linus' of git://git.kernel.dk/linux-2.6-block

* 'for-linus' of git://git.kernel.dk/linux-2.6-block:
  elevator: fix oops on early call to elevator_change()
---

diff --git a/block/elevator.c b/block/elevator.c
index 205b09a5bd9..4e11559aa2b 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 926b50322a4..4fd978e7eb8 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;
 };
 
 /*