ACPI/CPUIDLE: prevent setting pm_idle to NULL

pm_idle_save resp. pm_idle_old can be NULL when the restore code in
acpi_processor_cst_has_changed() resp. cpuidle_uninstall_idle_handler()
is called. This can set pm_idle unconditinally to NULL, which causes the
kernel to panic when calling pm_idle in the x86 idle code. This was
covered by an extra check for !pm_idle in the x86 idle code, which was
removed during the x86 idle code refactoring.

Instead of restoring the pm_idle check in the x86 code prevent the
acpi/cpuidle code to set pm_idle to NULL.

Reported by: Dhaval Giani http://lkml.org/lkml/2008/7/2/309
Based on a debug patch from Ingo Molnar

Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/acpi/processor_idle.c

···
 	if (!pr->flags.power_setup_done)
 		return -ENODEV;
 
-	/* Fall back to the default idle loop */
-	pm_idle = pm_idle_save;
-	synchronize_sched();	/* Relies on interrupts forcing exit from idle. */
+	/*
+	 * Fall back to the default idle loop, when pm_idle_save had
+	 * been initialized.
+	 */
+	if (pm_idle_save) {
+		pm_idle = pm_idle_save;
+		/* Relies on interrupts forcing exit from idle. */
+		synchronize_sched();
+	}
 
 	pr->flags.power = 0;
 	result = acpi_processor_get_power_info(pr);
···
 
 	/* Unregister the idle handler when processor #0 is removed. */
 	if (pr->id == 0) {
-		pm_idle = pm_idle_save;
+		if (pm_idle_save)
+			pm_idle = pm_idle_save;
 
 		/*
 		 * We are about to unload the current idle thread pm callback

drivers/cpuidle/cpuidle.c

···
  */
 void cpuidle_uninstall_idle_handler(void)
 {
-	if (enabled_devices && (pm_idle != pm_idle_old)) {
+	if (enabled_devices && pm_idle_old && (pm_idle != pm_idle_old)) {
 		pm_idle = pm_idle_old;
 		cpuidle_kick_cpus();
 	}