linux-kernel - Re: [RFC/PATCH] ftrace: add set_graph

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Date:	Fri, 11 Oct 2013 09:21:56 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [RFC/PATCH] ftrace: add set_graph_notrace filter

On Fri, Oct 11, 2013 at 12:17:17AM -0400, Steven Rostedt wrote:
> Isn't there a way we could pass the state? Hmm, I think we could use
> depth to do that. As depth is a pointer to trace.depth and not used
> before then. We could make it negative and then check that.
> 
> /me looks at other archs.
> 
> Darn it, s390 calls ftrace_push_return_trace() before
> ftrace_graph_entry(). They got fancy, as they must have noticed that
> trace.depth is set by ftrace_push_return_trace() and they just figured
> to let the ftrace_push_return_trace() do the work.
> 
> Hmm, we could add a config to do the check on one side or the other
> depending on how the arch handles it.
> 
> It needs to be well commented though.

I don't remember I why decided to do that more than four years ago, however
on the other hand I really don't think we need a config for this.
So I just committed the patch below... it compiles and doesn't crash
immediately, so must be good.
It will be merged during the next merge window.

>From 8c539d10400ee2efe000d324497a0661a2143dbf Mon Sep 17 00:00:00 2001
From: Heiko Carstens <heiko.carstens@...ibm.com>
Date: Fri, 11 Oct 2013 08:55:57 +0200
Subject: [PATCH] s390/ftrace: prepare_ftrace_return() function call order

Steven Rostedt noted that s390 is the only architecture which calls
ftrace_push_return_trace() before ftrace_graph_entry() and therefore has
the small advantage that trace.depth gets initialized automatically.

However this small advantage isn't worth the difference and possible subtle
breakage that may result from this.
So change s390 to have the same function call order like all other
architectures: first ftrace_graph_entry(), then ftrace_push_return_trace()

Reported-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
---
 arch/s390/kernel/ftrace.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index 1014ad5..224db03 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/arch/s390/kernel/ftrace.c
@@ -151,14 +151,13 @@ unsigned long __kprobes prepare_ftrace_return(unsigned long parent,
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		goto out;
 	ip = (ip & PSW_ADDR_INSN) - MCOUNT_INSN_SIZE;
-	if (ftrace_push_return_trace(parent, ip, &trace.depth, 0) == -EBUSY)
-		goto out;
 	trace.func = ip;
+	trace.depth = current->curr_ret_stack + 1;
 	/* Only trace if the calling function expects to. */
-	if (!ftrace_graph_entry(&trace)) {
-		current->curr_ret_stack--;
+	if (!ftrace_graph_entry(&trace))
+		goto out;
+	if (ftrace_push_return_trace(parent, ip, &trace.depth, 0) == -EBUSY)
 		goto out;
-	}
 	parent = (unsigned long) return_to_handler;
 out:
 	return parent;
-- 
1.8.3.4

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