ftrace: add ftrace_graph_stop()

Impact: new ftrace_graph_stop function

While developing more features of function graph, I hit a bug that
caused the WARN_ON to trigger in the prepare_ftrace_return function.
Well, it was hard for me to find out that was happening because the
bug would not print, it would just cause a hard lockup or reboot.
The reason is that it is not safe to call printk from this function.

Looking further, I also found that it calls unregister_ftrace_graph,
which grabs a mutex and calls kstop machine. This would definitely
lock the box up if it were to trigger.

This patch adds a fast and safe ftrace_graph_stop() which will
stop the function tracer. Then it is safe to call the WARN ON.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index afba918c623c..58ca1c3a3f4d 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -376,6 +376,8 @@ typedef void (*trace_func_graph_ent_t)(struct ftrace_graph_ent *); /* entry */
 extern int register_ftrace_graph(trace_func_graph_ret_t retfunc,
                                 trace_func_graph_ent_t entryfunc);
 
+extern void ftrace_graph_stop(void);
+
 /* The current handlers in use */
 extern trace_func_graph_ret_t ftrace_graph_return;
 extern trace_func_graph_ent_t ftrace_graph_entry;