linux-kernel - Re: [PATCH] ftrace: move function_stats out of _

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Date:	Mon, 4 Jul 2016 10:21:39 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Dmitry Safonov <0x7f454c46@...il.com>,
	linux-kernel@...r.kernel.org,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH] ftrace: move function_stats out of __initdata

On Mon,  4 Jul 2016 15:09:44 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> Not marking ftrace_init_tracefs() as __init causes a link time warning:
> 
> WARNING: vmlinux.o(.text+0xf5fcc): Section mismatch in reference from the function ftrace_init_tracefs() to the (unknown reference) .init.data:(unknown)
> 
> The problem here is that ftrace_init_tracefs() now gets called by
> init_tracer_tracefs(), which cannot be __init because it is called
> post init-time from instance_mkdir(). The function calls to the
> init-time ftrace_init_dyn_tracefs() and ftrace_profile_tracefs()
> functions apparently only happen for the initial call to
> ftrace_init_tracefs(), so we could safely ignore the warning, but
> at the same time, it all gets inlined anyway and we may as well
> remove the annotations.
> 
> The alternative would be restructuring the code so we only call
> trace_create_file() from init_tracer_tracefs(), but not the other
> init-time functions.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: 345ddcc882d8 ("ftrace: Have set_ftrace_pid use the bitmap like events do")

I know about this, and this is not the fix. The issue is that
ftrace_init_tracefs() has:

	if (tr->flags & TRACE_ARRAY_FL_GLOBAL) {
		ftrace_init_dyn_tracefs(d_tracer);
		ftrace_profile_tracefs(d_tracer);
	}

Where that condition is only true on boot up. Hence, those two
functions will never get called during normal runtime situations.

I have a patch to fix this, I just haven't gotten around to running it
through my test suite, or even committing it for that matter. It's a US
holiday today and I wont be doing any more with this till tomorrow.

Below is the patch. Feel free to test it out (I haven't done much with
it yet).

-- Steve

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 8b488f4dd8e8..84752c8e28b5 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5539,16 +5539,20 @@ static const struct file_operations ftrace_pid_fops = {
 
 void ftrace_init_tracefs(struct trace_array *tr, struct dentry *d_tracer)
 {
-	/* Only the top level directory has the dyn_tracefs and profile */
-	if (tr->flags & TRACE_ARRAY_FL_GLOBAL) {
-		ftrace_init_dyn_tracefs(d_tracer);
-		ftrace_profile_tracefs(d_tracer);
-	}
-
 	trace_create_file("set_ftrace_pid", 0644, d_tracer,
 			    tr, &ftrace_pid_fops);
 }
 
+void __init ftrace_init_tracefs_toplevel(struct trace_array *tr,
+					 struct dentry *d_tracer)
+{
+	/* Only the top level directory has the dyn_tracefs and profile */
+	WARN_ON(!(tr->flags & TRACE_ARRAY_FL_GLOBAL));
+
+	ftrace_init_dyn_tracefs(d_tracer);
+	ftrace_profile_tracefs(d_tracer);
+}
+
 /**
  * ftrace_kill - kill ftrace
  *
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 3d9f31b576f3..5fd53a7847bc 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7369,6 +7369,7 @@ static __init int tracer_init_tracefs(void)
 		return 0;
 
 	init_tracer_tracefs(&global_trace, d_tracer);
+	ftrace_init_tracefs_toplevel(&global_trace, d_tracer);
 
 	trace_create_file("tracing_thresh", 0644, d_tracer,
 			&global_trace, &tracing_thresh_fops);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index eaee458755a4..d751ed7c663b 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -857,6 +857,8 @@ void ftrace_init_array_ops(struct trace_array *tr, ftrace_func_t func);
 void ftrace_reset_array_ops(struct trace_array *tr);
 int using_ftrace_ops_list_func(void);
 void ftrace_init_tracefs(struct trace_array *tr, struct dentry *d_tracer);
+void ftrace_init_tracefs_toplevel(struct trace_array *tr,
+				  struct dentry *d_tracer);
 #else
 static inline int ftrace_trace_task(struct trace_array *tr)
 {