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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 17 May 2023 09:39:41 -0700
Message-ID: <CAHk-=wh+129KAcegJqvo2PKPUK3yw5qQwF0LcHofdT0bPbs8iQ@mail.gmail.com>
Subject: Re: [PATCH] tracing: make ftrace_likely_update() declaration visible
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        linux-trace-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Bart Van Assche <bvanassche@acm.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        linux-sparse@vger.kernel.org, linux-kernel@vger.kernel.org
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On Wed, May 17, 2023 at 5:47=E2=80=AFAM Arnd Bergmann <arnd@kernel.org> wro=
te:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> This function is only used when CONFIG_TRACE_BRANCH_PROFILING is
> set, and the declaration is hidden behind this Kconfig symbol,
> which causes a warning if disabled:

That explanation is not very clear.

The problem is that the tracing code itself is built with
DISABLE_BRANCH_PROFILING, in order to not recurse. And that hides the
declaration when the definition is compiled, leading to the problem.

CONFIG_TRACE_BRANCH_PROFILING isn't the problem in itself - *that*
part of the test is consistent (not used, not declared, and not
compiled if it is off).

The problem is that DISABLE_BRANCH_PROFILING case, where it is used
elsewhere, but not declared when it is itself compiled.

I applied the patch, but I tried to reword the explanation to be more
clear. I may have failed.

          Linus