kernel/notifier.c: double register detection

By design notifiers can be registerd once only, 2nd register attempt
called by mistake silently corrupts notifiers list.

A few years ago I investigated described problem, the host was power
cycled because of notifier list corruption.  I've prepared this patch
and applied it to the OpenVZ kernel and sent this patch but nobody
commented on it.  Later it helped us to detect a similar problem in the
OpenVz kernel.

Mistakes with notifier registration can happen for example during
subsystem initialization from different namespaces, or because of a lost
unregister in the roll-back path on initialization failures.

The proposed check cannot prevent the described problem, however it
allows us to detect its reason quickly without coredump analysis.

Link: http://lkml.kernel.org/r/04127e71-4782-9bbb-fe5a-7c01e93a99b0@virtuozzo.com
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/notifier.c

@@ -22,6 +22,7 @@ static int notifier_chain_register(struct notifier_block **nl,
 		struct notifier_block *n)
 {
 	while ((*nl) != NULL) {
+		WARN_ONCE(((*nl) == n), "double register detected");
 		if (n->priority > (*nl)->priority)
 			break;
 		nl = &((*nl)->next);