function-graph: add proper initialization for init task

Impact: fix to crash going to kexec

The init task did not properly initialize the function graph pointers.
Altough these pointers are NULL, they can not be assumed to be NULL
for the init task, and must still be properly initialize.

This usually is not an issue since a problem only arises when a task
exits, and the init tasks do not usually exit. But when doing tests
with kexec, the init tasks do exit, and the bug appears.

This patch properly initializes the init tasks function graph data
structures.
Reported-and-Tested-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <alpine.DEB.2.00.0903252053080.5675@gandalf.stny.rr.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/ftrace.h

@@ -356,6 +356,9 @@ struct ftrace_graph_ret {
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 
+/* for init task */
+#define INIT_FTRACE_GRAPH		.ret_stack = NULL
+
 /*
  * Stack of return addresses for functions
  * of a thread.
@@ -430,10 +433,11 @@ static inline void unpause_graph_tracing(void)
 {
 	atomic_dec(&current->tracing_graph_pause);
 }
-#else
+#else /* !CONFIG_FUNCTION_GRAPH_TRACER */
 
 #define __notrace_funcgraph
 #define __irq_entry
+#define INIT_FTRACE_GRAPH
 
 static inline void ftrace_graph_init_task(struct task_struct *t) { }
 static inline void ftrace_graph_exit_task(struct task_struct *t) { }
@@ -445,7 +449,7 @@ static inline int task_curr_ret_stack(struct task_struct *tsk)
 
 static inline void pause_graph_tracing(void) { }
 static inline void unpause_graph_tracing(void) { }
-#endif
+#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
 #ifdef CONFIG_TRACING
 #include <linux/sched.h>

include/linux/init_task.h

@@ -5,6 +5,7 @@
 #include <linux/irqflags.h>
 #include <linux/utsname.h>
 #include <linux/lockdep.h>
+#include <linux/ftrace.h>
 #include <linux/ipc.h>
 #include <linux/pid_namespace.h>
 #include <linux/user_namespace.h>
@@ -184,6 +185,7 @@ extern struct cred init_cred;
 	INIT_IDS							\
 	INIT_TRACE_IRQFLAGS						\
 	INIT_LOCKDEP							\
+	INIT_FTRACE_GRAPH						\
 }