debug: add notifier chain debugging

during some development we suspected a case where we left something
in a notifier chain that was from a module that was unloaded already...
and that sort of thing is rather hard to track down.

This patch adds a very simple sanity check (which isn't all that
expensive) to make sure the notifier we're about to call is
actually from either the kernel itself of from a still-loaded
module, avoiding a hard-to-chase-down crash.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

lib/Kconfig.debug

@@ -536,6 +536,16 @@ config DEBUG_SG
 
 	  If unsure, say N.
 
+config DEBUG_NOTIFIERS
+	bool "Debug notifier call chains"
+	depends on DEBUG_KERNEL
+	help
+	  Enable this to turn on sanity checking for notifier call chains.
+	  This is most useful for kernel developers to make sure that
+	  modules properly unregister themselves from notifier chains.
+	  This is a relatively cheap check but if you care about maximum
+	  performance, say N.
+
 config FRAME_POINTER
 	bool "Compile the kernel with frame pointers"
 	depends on DEBUG_KERNEL && \