linux-kernel - Re: [PATCH] tracing: make ftrace_likely

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Date:   Wed, 17 May 2023 09:39:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-trace-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Bart Van Assche <bvanassche@....org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: make ftrace_likely_update() declaration visible

On Wed, May 17, 2023 at 5:47 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.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