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, 6 insertions, 4 deletions

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 1a5b8f8..adba8e9 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -484,14 +484,16 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
 		: "memory"
 	);
 
-	if (WARN_ON(faulted)) {
-		unregister_ftrace_graph();
+	if (unlikely(faulted)) {
+		ftrace_graph_stop();
+		WARN_ON(1);
 		return;
 	}
 
-	if (WARN_ON(!__kernel_text_address(old))) {
-		unregister_ftrace_graph();
+	if (unlikely(!__kernel_text_address(old))) {
+		ftrace_graph_stop();
 		*parent = old;
+		WARN_ON(1);
 		return;
 	}