x86: Fix trigger_all_cpu_backtrace() implementation

The following change fixes the x86 implementation of
trigger_all_cpu_backtrace(), which was previously (accidentally,
as far as I can tell) disabled to always return false as on
architectures that do not implement this function.

trigger_all_cpu_backtrace(), as defined in include/linux/nmi.h,
should call arch_trigger_all_cpu_backtrace() if available, or
return false if the underlying arch doesn't implement this
function.

x86 did provide a suitable arch_trigger_all_cpu_backtrace()
implementation, but it wasn't actually being used because it was
declared in asm/nmi.h, which linux/nmi.h doesn't include. Also,
linux/nmi.h couldn't easily be fixed by including asm/nmi.h,
because that file is not available on all architectures.

I am proposing to fix this by moving the x86 definition of
arch_trigger_all_cpu_backtrace() to asm/irq.h.

Tested via: echo l > /proc/sysrq-trigger

Before the change, this uses a fallback implementation which
shows backtraces on active CPUs (using
smp_call_function_interrupt() )

After the change, this shows NMI backtraces on all CPUs

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1370518875-1346-1-git-send-email-walken@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/irq.h

@@ -41,4 +41,9 @@ extern int vector_used_by_percpu_irq(unsigned int vector);
 
 extern void init_ISA_irqs(void);
 
+#ifdef CONFIG_X86_LOCAL_APIC
+void arch_trigger_all_cpu_backtrace(void);
+#define arch_trigger_all_cpu_backtrace arch_trigger_all_cpu_backtrace
+#endif
+
 #endif /* _ASM_X86_IRQ_H */

arch/x86/include/asm/nmi.h

@@ -18,9 +18,7 @@ extern int proc_nmi_enabled(struct ctl_table *, int ,
 			void __user *, size_t *, loff_t *);
 extern int unknown_nmi_panic;
 
-void arch_trigger_all_cpu_backtrace(void);
-#define arch_trigger_all_cpu_backtrace arch_trigger_all_cpu_backtrace
-#endif
+#endif /* CONFIG_X86_LOCAL_APIC */
 
 #define NMI_FLAG_FIRST	1
 

arch/x86/kernel/apic/hw_nmi.c

@@ -9,6 +9,7 @@
  *
  */
 #include <asm/apic.h>
+#include <asm/nmi.h>
 
 #include <linux/cpumask.h>
 #include <linux/kdebug.h>