sysdev: detect multiple driver registrations

I've just found how easy it is to accidentally register a sysdev_driver for
two different classes.  When this happens, bad things happen as the
sysdev_driver structure keeps has the list entry for the driver
registration.

The following patch makes a WARN_ON() if this happens, although I think
BUG_ON or returning -EAGAIN could also be valid responses to this.
Signed-off-by: Ben Dooks <ben@fluff.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/base/sys.c

@@ -167,6 +167,22 @@ int sysdev_driver_register(struct sysdev_class *cls, struct sysdev_driver *drv)
 {
 	int err = 0;
 
+	if (!cls) {
+		printk(KERN_WARNING "sysdev: invalid class passed to "
+			"sysdev_driver_register!\n");
+		WARN_ON(1);
+		return -EINVAL;
+	}
+
+	/* Check whether this driver has already been added to a class. */
+	if ((drv->entry.next != drv->entry.prev) ||
+	    (drv->entry.next != NULL)) {
+		printk(KERN_WARNING "sysdev: class %s: driver (%p) has already"
+			" been registered to a class, something is wrong, but "
+			"will forge on!\n", cls->name, drv);
+		WARN_ON(1);
+	}
+
 	mutex_lock(&sysdev_drivers_lock);
 	if (cls && kset_get(&cls->kset)) {
 		list_add_tail(&drv->entry, &cls->drivers);