cpumask: fix CONFIG_CPUMASK_OFFSTACK=y cpu hotunplug crash

Impact: Fix cpu offline when CONFIG_MAXSMP=y

Changeset bc9b83dd "cpumask: convert
c1e_mask in arch/x86/kernel/process.c to cpumask_var_t" contained a
bug: c1e_mask is manipulated even if C1E isn't detected (and hence
not allocated).

This is simply fixed by checking for NULL (which gcc optimizes out
anyway of CONFIG_CPUMASK_OFFSTACK=n, since it knows ce1_mask can never
be NULL).

In addition, fix a leak where select_idle_routine re-allocates
(and re-clears) c1e_mask on every cpu init.
Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Mike Travis <travis@sgi.com>
LKML-Reference: <200903171450.34549.rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/processor.h

@@ -733,6 +733,7 @@ static inline void __sti_mwait(unsigned long eax, unsigned long ecx)
 extern void mwait_idle_with_hints(unsigned long eax, unsigned long ecx);
 
 extern void select_idle_routine(const struct cpuinfo_x86 *c);
+extern void init_c1e_mask(void);
 
 extern unsigned long		boot_option_idle_override;
 extern unsigned long		idle_halt;

arch/x86/kernel/cpu/common.c

@@ -812,6 +812,7 @@ static void vgetcpu_set_mode(void)
 void __init identify_boot_cpu(void)
 {
 	identify_cpu(&boot_cpu_data);
+	init_c1e_mask();
 #ifdef CONFIG_X86_32
 	sysenter_setup();
 	enable_sep_cpu();

arch/x86/kernel/process.c

@@ -479,7 +479,8 @@ static int c1e_detected;
 
 void c1e_remove_cpu(int cpu)
 {
-	cpumask_clear_cpu(cpu, c1e_mask);
+	if (c1e_mask != NULL)
+		cpumask_clear_cpu(cpu, c1e_mask);
 }
 
 /*
@@ -556,13 +557,20 @@ void __cpuinit select_idle_routine(const struct cpuinfo_x86 *c)
 		pm_idle = mwait_idle;
 	} else if (check_c1e_idle(c)) {
 		printk(KERN_INFO "using C1E aware idle routine\n");
-		alloc_cpumask_var(&c1e_mask, GFP_KERNEL);
-		cpumask_clear(c1e_mask);
 		pm_idle = c1e_idle;
 	} else
 		pm_idle = default_idle;
 }
 
+void __init init_c1e_mask(void)
+{
+	/* If we're using c1e_idle, we need to allocate c1e_mask. */
+	if (pm_idle == c1e_idle) {
+		alloc_cpumask_var(&c1e_mask, GFP_KERNEL);
+		cpumask_clear(c1e_mask);
+	}
+}
+
 static int __init idle_setup(char *str)
 {
 	if (!str)