[PATCH] block: fix blk_register_queue() return value

[Akinobu Mita <akinobu.mita@gmail.com>]

blk_register_queue() returns -ENXIO when queue->request_fn is NULL.
But there are some block drivers that call blk_register_queue()
via add_disk() with queue->request_fn == NULL. (For example, brd, loop)

Although no one checks return value of blk_register_queue(), this patch
makes it return 0 instead of -ENXIO when queue->request_fn is NULL,

Also this patch adds warning when blk_register_queue() and
blk_unregister_queue() are called with queue == NULL rather than
ignore invalid usage silently.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
---
 block/blk-sysfs.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Index: 2.6-rc/block/blk-sysfs.c
===================================================================
--- 2.6-rc.orig/block/blk-sysfs.c
+++ 2.6-rc/block/blk-sysfs.c
@@ -276,9 +276,12 @@ int blk_register_queue(struct gendisk *d
 
 	struct request_queue *q = disk->queue;
 
-	if (!q || !q->request_fn)
+	if (WARN_ON(!q))
 		return -ENXIO;
 
+	if (!q->request_fn)
+		return 0;
+
 	ret = kobject_add(&q->kobj, kobject_get(&disk->dev.kobj),
 			  "%s", "queue");
 	if (ret < 0)
@@ -300,7 +303,10 @@ void blk_unregister_queue(struct gendisk
 {
 	struct request_queue *q = disk->queue;
 
-	if (q && q->request_fn) {
+	if (WARN_ON(!q))
+		return;
+
+	if (q->request_fn) {
 		elv_unregister_queue(q);
 
 		kobject_uevent(&q->kobj, KOBJ_REMOVE);