linux-kernel - Re: kernel/trace/ftrace.c:7157:20: error: unused function 'ftrace_startup

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Message-ID: <CAK7LNAQmztEn7nN_R05zhXWc-5S44Y5nGQQjzdByTPxgDH13wA@mail.gmail.com>
Date:   Tue, 15 Feb 2022 01:28:53 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        kernel test robot <lkp@...el.com>,
        chongjiapeng <jiapeng.chong@...ux.alibaba.com>,
        llvm@...ts.linux.dev, kbuild-all@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kernel/trace/ftrace.c:7157:20: error: unused function 'ftrace_startup_enable'

On Tue, Feb 15, 2022 at 12:53 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Hi Steve,
>
> On Mon, Feb 14, 2022 at 10:20:00AM -0500, Steven Rostedt wrote:
> > On Sun, 13 Feb 2022 21:03:29 +0800
> > kernel test robot <lkp@...el.com> wrote:
> >
> > > All errors (new ones prefixed by >>):
> > >
> > > >> kernel/trace/ftrace.c:7157:20: error: unused function 'ftrace_startup_enable' [-Werror,-Wunused-function]
> > >    static inline void ftrace_startup_enable(int command) { }
> > >                       ^
> > >    1 error generated.
> >
> > Strange. I always thought that static inline functions do not cause
> > warnings when not used? Especially, since they are often in headers when
> > things are turned off. Or is it because this is in a C file?
>
> With -Wunused-function, clang will warn about unused static inline
> functions within a .c file (but not .h), whereas GCC will not warn for
> either. The unused attribute was added to the definition of inline to
> make clang's behavior match GCC's.
>
> > Is this a new warning caused by a commit, or is it a new warning because
> > the compiler now complains about it?
>
> However, in commit 6863f5643dd7 ("kbuild: allow Clang to find unused
> static inline functions for W=1 build"), Masahiro made it so that the
> unused attribute does not get added at W=1 so that instances of unused
> static inline functions can be caught and eliminated (or put into use,
> if the function should have been used), hence this report.

BTW, my hope was to move this warning to W=0 someday.
(that is, remove __inline_maybe_unused entirely).

I do not know how many warnings are still remaining, though.
Is it now more difficult due to CONFIG_WERROR?



> I will be honest, I don't know why the robot flagged 172f7ba9772c as the
> commit that introduced this warning but it seems legitimate if
> CONFIG_DYNAMIC_FTRACE is not enabled, since ftrace_startup_enable() is
> only ever used within an '#ifdef CONFIG_DYNAMIC_FTRACE' block so I guess
> the stub is unnecessary?
>
> Cheers,
> Nathan



-- 
Best Regards
Masahiro Yamada