[PATCH] x86, x86_64: Only handle system NMIs on the BSP

 While discussing races in the NMI handler's trailer fiddling with RTC
registers, I've discovered we incorrectly attempt to handle NMIs coming
from the system (memory errors, IOCHK# assertions, etc.) with all
processors even though the interrupts are only routed to the bootstrap
processor.  If one of these events coincides with a NMI watchdog tick it
may even be handled multiple times in parallel.

Here is a fix that makes application processors ignore these events.  They
no longer access the NMI status bits at I/O port 0x61 which has also the
advantage of removing the contention on the port when the I/O APIC NMI
watchdog makes all processors arrive at the handler at the same time.
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/traps.c

@@ -583,7 +583,11 @@ void die_nmi (struct pt_regs *regs, const char *msg)
 
 static void default_do_nmi(struct pt_regs * regs)
 {
-	unsigned char reason = get_nmi_reason();
+	unsigned char reason = 0;
+
+	/* Only the BSP gets external NMIs from the system.  */
+	if (!smp_processor_id())
+		reason = get_nmi_reason();
  
 	if (!(reason & 0xc0)) {
 		if (notify_die(DIE_NMI_IPI, "nmi_ipi", regs, reason, 0, SIGINT)

arch/x86_64/kernel/traps.c

@@ -580,9 +580,13 @@ static void unknown_nmi_error(unsigned char reason, struct pt_regs * regs)
 	printk("Do you have a strange power saving mode enabled?\n");
 }
 
-asmlinkage void default_do_nmi(struct pt_regs * regs)
+asmlinkage void default_do_nmi(struct pt_regs *regs)
 {
-	unsigned char reason = get_nmi_reason();
+	unsigned char reason;
+
+	/* Only the BSP gets external NMIs from the system.  */
+	if (!smp_processor_id())
+		reason = get_nmi_reason();
 
 	if (!(reason & 0xc0)) {
 		if (notify_die(DIE_NMI_IPI, "nmi_ipi", regs, reason, 0, SIGINT)