[PATCH] hotplug: Allow modules to use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU

The following patchset allows a host with running virtual machines to be
suspended and, on at least a subset of the machines tested, resumed.  Note
that this is orthogonal to suspending and resuming an individual guest to a
file.

A side effect of implementing suspend/resume is that cpu hotplug is now
supported.  This should please the owners of big iron.

This patch:

KVM wants the cpu hotplug notifications, both for cpu hotplug itself, but more
commonly for host suspend/resume.

In order to avoid extensive #ifdefs, provide stubs when CONFIG_CPU_HOTPLUG is
not defined.

In all, we have four cases:

- UP: register and unregister stubbed out
- SMP+hotplug: full register and unregister
- SMP, no hotplug, core: register as __init, unregister stubbed
      (cpus are brought up during core initialization)
- SMP, no hotplug, module: register and unregister stubbed out
      (cpus cannot be brought up during module lifetime)

Signed-off-by: Avi Kivity <avi@qumranet.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cpu.h

@@ -49,10 +49,20 @@ struct notifier_block;
 
 #ifdef CONFIG_SMP
 /* Need to know about CPUs going up/down? */
-extern int register_cpu_notifier(struct notifier_block *nb);
 #ifdef CONFIG_HOTPLUG_CPU
+extern int register_cpu_notifier(struct notifier_block *nb);
 extern void unregister_cpu_notifier(struct notifier_block *nb);
 #else
+
+#ifndef MODULE
+extern int register_cpu_notifier(struct notifier_block *nb);
+#else
+static inline int register_cpu_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
+#endif
+
 static inline void unregister_cpu_notifier(struct notifier_block *nb)
 {
 }