On Wed, 17 May 2023 09:39:41 -0700
Linus Torvalds [off-list ref] wrote:

On Wed, May 17, 2023 at 5:47 AM Arnd Bergmann [off-list ref] wrote:

quoted

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.

That's pretty much it. The DISABLE_BRANCH_PROFILING is to disable it in
various places where it will crash if used (vdso is one of them). It is
also used to prevent the code that declares it from recursing on itself (as
you stated).

Thus, the issue is simply that the declaration is hidden by the recursion
protection where the function is actually defined.

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve