tracing/function-return-tracer: store return stack into task_struct and allocate it dynamically

Impact: use deeper function tracing depth safely

Some tests showed that function return tracing needed a more deeper depth
of function calls. But it could be unsafe to store these return addresses
to the stack.

So these arrays will now be allocated dynamically into task_struct of current
only when the tracer is activated.

Typical scheme when tracer is activated:
- allocate a return stack for each task in global list.
- fork: allocate the return stack for the newly created task
- exit: free return stack of current
- idle init: same as fork

I chose a default depth of 50. I don't have overruns anymore.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/exit.c

@@ -47,6 +47,7 @@
 #include <linux/task_io_accounting_ops.h>
 #include <linux/tracehook.h>
 #include <trace/sched.h>
+#include <linux/ftrace.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -1127,7 +1128,9 @@ NORET_TYPE void do_exit(long code)
 	preempt_disable();
 	/* causes final put_task_struct in finish_task_switch(). */
 	tsk->state = TASK_DEAD;
-
+#ifdef CONFIG_FUNCTION_RET_TRACER
+	ftrace_retfunc_exit_task(tsk);
+#endif
 	schedule();
 	BUG();
 	/* Avoid "noreturn function does return".  */