linux-kernel - Re: [BUG] tracing: dynamic ftrace selftest detected failures

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Message-ID: <20240821115000.387de298@gandalf.local.home>
Date: Wed, 21 Aug 2024 11:50:00 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Sami Tolvanen
 <samitolvanen@...gle.com>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, linux-kernel@...r.kernel.org,
 clang-built-linux <llvm@...ts.linux.dev>, Nathan Chancellor
 <nathan@...nel.org>
Subject: Re: [BUG] tracing: dynamic ftrace selftest detected failures

On Wed, 21 Aug 2024 16:42:07 +0100
Mark Rutland <mark.rutland@....com> wrote:

> FWIW, that was in samples/ftrace/ftrace-ops.c, where tracee_relevant() and
> tracee_irrelevant() have the barrier():
> 
> | /*
> |  * Marked as noinline to ensure that an out-of-line traceable copy is
> |  * generated by the compiler.
> |  *
> |  * The barrier() ensures the compiler won't elide calls by determining there
> |  * are no side-effects.
> |  */
> | static noinline void tracee_relevant(void)
> | {
> |         barrier();
> | }
> 
> ... so we already have precedent for that in tracing code.

If adding a barrier() fixes things, then I'm 100% for adding them.

Masami, does this fix it?

-- Steve

diff --git a/kernel/trace/trace_selftest_dynamic.c b/kernel/trace/trace_selftest_dynamic.c
index c364cf777e1a..a112433d78ce 100644
--- a/kernel/trace/trace_selftest_dynamic.c
+++ b/kernel/trace/trace_selftest_dynamic.c
@@ -4,12 +4,20 @@
 
 noinline __noclone int DYN_FTRACE_TEST_NAME(void)
 {
-	/* used to call mcount */
+	/*
+	 * Used to test function tracing. Make sure the compiler doesn't
+	 * remove it.
+	 */
+	barrier();
 	return 0;
 }
 
 noinline __noclone int DYN_FTRACE_TEST_NAME2(void)
 {
-	/* used to call mcount */
+	/*
+	 * Used to test function tracing. Make sure the compiler doesn't
+	 * remove it.
+	 */
+	barrier();
 	return 0;
 }