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

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Message-ID: <ZsYInqSc-WS4UldP@J2N7QTR9R3.cambridge.arm.com>
Date: Wed, 21 Aug 2024 16:32:46 +0100
From: Mark Rutland <mark.rutland@....com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	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, Aug 21, 2024 at 07:05:39AM +0900, Masami Hiramatsu wrote:
> On Tue, 20 Aug 2024 08:10:42 -0700
> Sami Tolvanen <samitolvanen@...gle.com> wrote:
> 
> > On Tue, Aug 20, 2024 at 3:48 AM Mark Rutland <mark.rutland@....com> wrote:
> > >
> > > On Tue, Aug 20, 2024 at 10:03:30AM +0900, Masami Hiramatsu wrote:
> > > > On Mon, 19 Aug 2024 12:02:44 -0400
> > > > Steven Rostedt <rostedt@...dmis.org> wrote:
> > > >
> > > > > On Tue, 20 Aug 2024 00:56:49 +0900
> > > > > Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> > > > > >
> > > > > > >
> > > > > > > We may need to add "noinline" or something to make sure those functions
> > > > > > > don't get inlined for LTO.
> > > > > >
> > > > > > Yeah, we need such option at least for function call test.
> > > > >
> > > > > Could you add the noinline, and if it fixes the issue send a patch?
> > > >
> > > > I found the target function already has "noinline". I tried to add noinline
> > > > to the testing function (callsite), but it also did not work.
> > > > I think "noinline" is for the compiler, but LTO is done by the linker.
> > >
> > > If LTO is breaking noinline, then that has much larger implications for
> > > noinstr code and similar, and means that LTO is unsound...
> > 
> > The noinline attribute is preserved in LLVM IR, so it should continue
> > to work with LTO. Which function are we talking about here? Are you
> > sure the function was inlined instead of being dropped completely?
> > Does marking the function __used help?
> 
> We are talking about trace_selftest_startup_dynamic_tracing() in
> kernel/trace/trace_selftest.c. The callee is func() which is actually
> DYN_FTRACE_TEST_NAME() in kernel/trace/trace_selftest_dynamic.c.
> That function passed as pointer (but the compiler can embed it by constant
> propagation.)

Ah, so IIUC the function isn't being inlined; the call is being
optimized away becase callee() has no side-effects.

That can happen without LTO if the caller is in the same compilation
unit, and I have worked around that in the past by adding a barrier()
into the callee.

If you add a barrier() into the callee, does it work? I suspect that's a
reasonable workaround.

Mark.