nmi: add generic nmi tracking state

This code adds an in_nmi() macro that uses the current tasks preempt count
to track when it is in NMI context. Other parts of the kernel can
use this to determine if the context is in NMI context or not.

This code was inspired by the -rt patch in_nmi version that was
written by Peter Zijlstra, who borrowed that code from
Mathieu Desnoyers.
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>

include/linux/hardirq.h

@@ -61,6 +61,12 @@
 #error PREEMPT_ACTIVE is too low!
 #endif
 
+#define NMI_OFFSET	(PREEMPT_ACTIVE << 1)
+
+#if NMI_OFFSET >= 0x80000000
+#error PREEMPT_ACTIVE too high!
+#endif
+
 #define hardirq_count()	(preempt_count() & HARDIRQ_MASK)
 #define softirq_count()	(preempt_count() & SOFTIRQ_MASK)
 #define irq_count()	(preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK))
@@ -73,6 +79,11 @@
 #define in_softirq()		(softirq_count())
 #define in_interrupt()		(irq_count())
 
+/*
+ * Are we in NMI context?
+ */
+#define in_nmi()	(preempt_count() & NMI_OFFSET)
+
 #if defined(CONFIG_PREEMPT)
 # define PREEMPT_INATOMIC_BASE kernel_locked()
 # define PREEMPT_CHECK_OFFSET 1
@@ -167,6 +178,8 @@ extern void irq_exit(void);
 #define nmi_enter()				\
 	do {					\
 		ftrace_nmi_enter();		\
+		BUG_ON(in_nmi());		\
+		add_preempt_count(NMI_OFFSET);	\
 		lockdep_off();			\
 		rcu_nmi_enter();		\
 		__irq_enter();			\
@@ -177,6 +190,8 @@ extern void irq_exit(void);
 		__irq_exit();			\
 		rcu_nmi_exit();			\
 		lockdep_on();			\
+		BUG_ON(!in_nmi());		\
+		sub_preempt_count(NMI_OFFSET);	\
 		ftrace_nmi_exit();		\
 	} while (0)