sh: Mark __switch_to() as __notrace_funcgraph

Annotate __switch_to() so that the function graph tracer does not try to
trace it. Use __notrace_funcgraph, as opposed to notrace, so that other
tracers can continue to trace __switch_to().

The reason that we don't want to trace __switch_to() with the function
graph tracer is because of how the return address stack in task_struct
is implemented. When we enter __switch_to we store the real return
address on prev's ret_stack. When we return from __switch_to() we've
patched the return address on the kernel stack to be
return_to_handler. Calling return_to_handler we do,

       -> ftrace_return_to_handler()
       	  -> ftrace_pop_return_ftrace()

Which tries to pop the real return address from current->ret_stack. The
problem being that we stored the return address on prev->ret_stack, but
current now points to next, and next->ret_stack doesn't contain the
correct return address (and is possibly even empty).
Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/kernel/process_32.c

@@ -23,6 +23,7 @@
 #include <linux/tick.h>
 #include <linux/reboot.h>
 #include <linux/fs.h>
+#include <linux/ftrace.h>
 #include <linux/preempt.h>
 #include <asm/uaccess.h>
 #include <asm/mmu_context.h>
@@ -264,8 +265,8 @@ static void ubc_set_tracing(int asid, unsigned long pc)
  *	switch_to(x,y) should switch tasks from x to y.
  *
  */
-struct task_struct *__switch_to(struct task_struct *prev,
-				struct task_struct *next)
+__notrace_funcgraph struct task_struct *
+__switch_to(struct task_struct *prev, struct task_struct *next)
 {
 #if defined(CONFIG_SH_FPU)
 	unlazy_fpu(prev, task_pt_regs(prev));