[PATCH] CPU hotplug: call check_tsc_sync_source() with irqs off

check_tsc_sync_source() depends on being called with irqs disabled (it
checks whether the TSC is coherent across two specific CPUs). This is
incidentally true during bootup, but not during cpu hotplug __cpu_up().
This got found via smp_processor_id() debugging.

disable irqs explicitly and remove the unconditional enabling of
interrupts. Add touch_nmi_watchdog() to the cpu_online_map busy loop.

this bug is present both on i386 and on x86_64.
Reported-by: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/i386/kernel/smpboot.c

@@ -45,6 +45,7 @@
 #include <linux/notifier.h>
 #include <linux/cpu.h>
 #include <linux/percpu.h>
+#include <linux/nmi.h>
 
 #include <linux/delay.h>
 #include <linux/mc146818rtc.h>
@@ -1278,8 +1279,9 @@ void __cpu_die(unsigned int cpu)
 
 int __cpuinit __cpu_up(unsigned int cpu)
 {
+	unsigned long flags;
 #ifdef CONFIG_HOTPLUG_CPU
-	int ret=0;
+	int ret = 0;
 
 	/*
 	 * We do warm boot only on cpus that had booted earlier
@@ -1297,23 +1299,25 @@ int __cpuinit __cpu_up(unsigned int cpu)
 	/* In case one didn't come up */
 	if (!cpu_isset(cpu, cpu_callin_map)) {
 		printk(KERN_DEBUG "skipping cpu%d, didn't come online\n", cpu);
-		local_irq_enable();
 		return -EIO;
 	}
 
-	local_irq_enable();
-
 	per_cpu(cpu_state, cpu) = CPU_UP_PREPARE;
 	/* Unleash the CPU! */
 	cpu_set(cpu, smp_commenced_mask);
 
 	/*
-	 * Check TSC synchronization with the AP:
+	 * Check TSC synchronization with the AP (keep irqs disabled
+	 * while doing so):
 	 */
+	local_irq_save(flags);
 	check_tsc_sync_source(cpu);
+	local_irq_restore(flags);
 
-	while (!cpu_isset(cpu, cpu_online_map))
+	while (!cpu_isset(cpu, cpu_online_map)) {
 		cpu_relax();
+		touch_nmi_watchdog();
+	}
 
 #ifdef CONFIG_X86_GENERICARCH
 	if (num_online_cpus() > 8 && genapic == &apic_default)

arch/x86_64/kernel/smpboot.c

@@ -923,8 +923,9 @@ void __init smp_prepare_boot_cpu(void)
  */
 int __cpuinit __cpu_up(unsigned int cpu)
 {
-	int err;
 	int apicid = cpu_present_to_apicid(cpu);
+	unsigned long flags;
+	int err;
 
 	WARN_ON(irqs_disabled());
 
@@ -958,7 +959,9 @@ int __cpuinit __cpu_up(unsigned int cpu)
 	/*
   	 * Make sure and check TSC sync:
  	 */
+	local_irq_save(flags);
 	check_tsc_sync_source(cpu);
+	local_irq_restore(flags);
 
 	while (!cpu_isset(cpu, cpu_online_map))
 		cpu_relax();