linux-kernel - [PATCH 1/5] hotplug: Allow modules to use the cpu hotplug notifiers even if !CONFIG_HOTPLUG

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Message-Id: <20070130145316.DC710A0014@il.qumranet.com>
Date:	Tue, 30 Jan 2007 14:53:16 -0000
From:	Avi Kivity <avi@...ranet.com>
To:	kvm-devel@...ts.sourceforge.net
Cc:	linux-kernel@...r.kernel.org, akpm@...l.org, mingo@...e.hu
Subject: [PATCH 1/5] hotplug: Allow modules to use the cpu hotplug notifiers
	even if !CONFIG_HOTPLUG_CPU

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@...ranet.com>

Index: linux-2.6/include/linux/cpu.h
===================================================================
--- linux-2.6.orig/include/linux/cpu.h
+++ linux-2.6/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)
 {
 }
-
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