On Wed, Aug 21, 2024 at 07:05:39AM +0900, Masami Hiramatsu wrote: > On Tue, 20 Aug 2024 08:10:42 -0700 > Sami Tolvanen <samitolva...@google.com> wrote: > > > On Tue, Aug 20, 2024 at 3:48 AM Mark Rutland <mark.rutl...@arm.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 <rost...@goodmis.org> wrote: > > > > > > > > > On Tue, 20 Aug 2024 00:56:49 +0900 > > > > > Masami Hiramatsu (Google) <mhira...@kernel.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.
