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

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Message-Id: <20240823090412.d63468ab89ba28415d7bc649@kernel.org>
Date: Fri, 23 Aug 2024 09:04:12 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mark Rutland <mark.rutland@....com>, 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 11:50:00 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> 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?

Interesting, this fixes the issue!
And it is great to find how to avoid the LTO removes function like this.

Tested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> 
> -- 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;
>  }


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>