ftrace: change function graph tracer to use new in_nmi

The function graph tracer piggy backed onto the dynamic ftracer
to use the in_nmi custom code for dynamic tracing. The problem
was (as Andrew Morton pointed out) it really only wanted to bail
out if the context of the current CPU was in NMI context. But the
dynamic ftrace in_nmi custom code was true if _any_ CPU happened
to be in NMI context.

Now that we have a generic in_nmi interface, this patch changes
the function graph code to use it instead of the dynamic ftarce
custom code.

Reported-by: Andrew Morton <[e-mail]>
Signed-off-by: Steven Rostedt <[e-mail]>

arch/x86/Kconfig

···
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACE_MCOUNT_TEST
-	select HAVE_FTRACE_NMI_ENTER if DYNAMIC_FTRACE || FUNCTION_GRAPH_TRACER
+	select HAVE_FTRACE_NMI_ENTER if DYNAMIC_FTRACE
 	select HAVE_KVM if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64)
 	select HAVE_ARCH_KGDB if !X86_VOYAGER
 	select HAVE_ARCH_TRACEHOOK

arch/x86/kernel/ftrace.c

···
 	return ftrace_mod_jmp(ip, old_offset, new_offset);
 }
 
-#else /* CONFIG_DYNAMIC_FTRACE */
-
-/*
- * These functions are picked from those used on
- * this page for dynamic ftrace. They have been
- * simplified to ignore all traces in NMI context.
- */
-static atomic_t nmi_running;
-
-void arch_ftrace_nmi_enter(void)
-{
-	atomic_inc(&nmi_running);
-}
-
-void arch_ftrace_nmi_exit(void)
-{
-	atomic_dec(&nmi_running);
-}
-
 #endif /* !CONFIG_DYNAMIC_FTRACE */
 
 /* Add a function return address to the trace stack on thread info.*/
···
 				&return_to_handler;
 
 	/* Nmi's are currently unsupported */
-	if (unlikely(atomic_read(&nmi_running)))
+	if (unlikely(in_nmi()))
 		return;
 
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))