[PATCH] uml: use always a separate io thread for UBD

Currently, ubd=sync is different from replacing ubd#= with ubd#s=.  This is
against Principle of Least Surprise, so remove this difference.

Also the current ubd=sync behaviour is completely useless: it is to make sure
that when the kernel has synched its I/O to the virtual disk, the host does
not invalidate this with his caching; this causes ReiserFS corruption.

But since actually we call end_request() only after the io_thread has done its
work, we never lie to the block layer.  Using O_SYNC as we do when replacing
ubd#= with ubd#s= is enough.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/drivers/ubd_kern.c

@@ -773,9 +773,11 @@ int ubd_driver_init(void){
 	unsigned long stack;
 	int err;
 
+	/* Set by CONFIG_BLK_DEV_UBD_SYNC or ubd=sync.*/
 	if(global_openflags.s){
-		printk(KERN_INFO "ubd : Synchronous mode\n");
-		return(0);
+		printk(KERN_INFO "ubd: Synchronous mode\n");
+		/* Letting ubd=sync be like using ubd#s= instead of ubd#= is
+		 * enough. So use anyway the io thread. */
 	}
 	stack = alloc_stack(0, 0);
 	io_pid = start_io_thread(stack + PAGE_SIZE - sizeof(void *),