linux-kernel - Re: [PATCH] xen/trace: avoid clang warning on function pointers

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Message-ID: <CA+icZUVJsTMmmUY5i9=hZe9u4bxGgEG03Of5Xkh-QVvoHZW50A@mail.gmail.com>
Date:   Fri, 12 Jul 2019 11:16:35 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Petr Mladek <pmladek@...e.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] xen/trace: avoid clang warning on function pointers

On Fri, Jul 12, 2019 at 10:59 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> clang-9 does not like the way that the is_signed_type() compares
> function pointers deep inside of the trace even macros:
>
> In file included from arch/x86/xen/trace.c:21:
> In file included from include/trace/events/xen.h:475:
> In file included from include/trace/define_trace.h:102:
> In file included from include/trace/trace_events.h:467:
> include/trace/events/xen.h:69:7: error: ordered comparison of function pointers ('xen_mc_callback_fn_t' (aka 'void (*)(void *)') and 'xen_mc_callback_fn_t') [-Werror,-Wordered-compare-function-pointers]
>                     __field(xen_mc_callback_fn_t, fn)
>                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> include/trace/trace_events.h:415:29: note: expanded from macro '__field'
>  #define __field(type, item)     __field_ext(type, item, FILTER_OTHER)
>                                 ^
> include/trace/trace_events.h:401:6: note: expanded from macro '__field_ext'
>                                  is_signed_type(type), filter_type);    \
>                                  ^
> include/linux/trace_events.h:540:44: note: expanded from macro 'is_signed_type'
>  #define is_signed_type(type)    (((type)(-1)) < (type)1)
>                                               ^
> note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
> include/trace/trace_events.h:77:16: note: expanded from macro 'TRACE_EVENT'
>                              PARAMS(tstruct),                  \
>                              ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> include/linux/tracepoint.h:95:25: note: expanded from macro 'PARAMS'
>  #define PARAMS(args...) args
>                         ^
> include/trace/trace_events.h:455:2: note: expanded from macro 'DECLARE_EVENT_CLASS'
>         tstruct;                                                        \
>         ^~~~~~~
>
> I guess the warning is reasonable in principle, though this seems to
> be the only instance we get in the entire kernel today.
> Shut up the warning by making it a void pointer in the exported
> structure.
>

Thanks for bringing this up (again), Arnd.

As this is a known CBL issue please add...

Link: https://github.com/ClangBuiltLinux/linux/issues/97

...and...

Tested-by: Sedat Dilek <sedat.dilek@...il.com>

For the sake of completeness see also the comments of Steven Rostedt
and user "Honeybyte" in the above Link - if not known/read.

- Sedat -

P.S.: I am using this patch since 6 months in my
for-5.x/clang-warningfree local Git repository.

> Fixes: c796f213a693 ("xen/trace: add multicall tracing")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/trace/events/xen.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/trace/events/xen.h b/include/trace/events/xen.h
> index 9a0e8af21310..f75b77414ac1 100644
> --- a/include/trace/events/xen.h
> +++ b/include/trace/events/xen.h
> @@ -66,7 +66,7 @@ TRACE_EVENT(xen_mc_callback,
>             TP_PROTO(xen_mc_callback_fn_t fn, void *data),
>             TP_ARGS(fn, data),
>             TP_STRUCT__entry(
> -                   __field(xen_mc_callback_fn_t, fn)
> +                   __field(void *, fn)
>                     __field(void *, data)
>                     ),
>             TP_fast_assign(
> --
> 2.20.0
>
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