[PATCH 1/5] ftrace: add ftrace_graph_stop

[Steven Rostedt <rostedt@goodmis.org>]

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From: Steven Rostedt <srostedt@redhat.com>

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>
---
 arch/x86/kernel/ftrace.c |   10 ++++++----
 include/linux/ftrace.h   |    2 ++
 kernel/trace/ftrace.c    |    5 +++++
 3 files changed, 13 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;
 	}
 
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index afba918..58ca1c3 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;
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 2e78628..a44af05 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1769,5 +1769,10 @@ void ftrace_graph_exit_task(struct task_struct *t)
 
 	kfree(ret_stack);
 }
+
+void ftrace_graph_stop(void)
+{
+	ftrace_stop();
+}
 #endif
 
-- 
1.5.6.5

--