[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>

1 files changed, 6 insertions, 4 deletions

diff --git a/block/elevator.c b/block/elevator.c
index dbbea73a8b10..96a61e029ce5 100644
--- a/block/elevator.c
+++ b/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)
-                        printk("I/O scheduler %s not found\n", 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);