[BLOCK] elevator: allow default scheduler to potentially be modular

Jens has decided that allowing the default scheduler to be a module is
a bug, and should not be allowed under kconfig.  However, I find that
scenario useful for debugging, and wish for the kernel to be able to
handle this situation without OOPSing, if I enable such an option in
the .config directly.  This patch dynamically checks for the presence
of the compiled-in default, and falls back to no-op, emitting a
suitable error message, when the default is not available

Tested for a range of boot options on 2.6.16-rc1-mm2.
Signed-off-by: Nate Diller <nate.diller@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

block/elevator.c

@@ -168,10 +168,12 @@ int elevator_init(request_queue_t *q, char *name)
 	if (name && !(e = elevator_get(name)))
 		return -EINVAL;
 
-	if (!e && !(e = elevator_get(chosen_elevator))) {
-		e = elevator_get(CONFIG_DEFAULT_IOSCHED);
-		if (*chosen_elevator)
+	if (!e && *chosen_elevator && !(e = elevator_get(chosen_elevator)))
 		printk("I/O scheduler %s not found\n", chosen_elevator);
+
+	if (!e && !(e = elevator_get(CONFIG_DEFAULT_IOSCHED))) {
+		printk("Default I/O scheduler not found, using no-op\n");
+		e = elevator_get("noop");
 	}
 
 	eq = kmalloc(sizeof(struct elevator_queue), GFP_KERNEL);