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

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Date:   Fri, 12 Jul 2019 10:41:42 -0700
From:   Nathan Chancellor <natechancellor@...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@...glegroups.com
Subject: Re: [PATCH] xen/trace: avoid clang warning on function pointers

On Fri, Jul 12, 2019 at 10:58:48AM +0200, Arnd Bergmann 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.
> 
> Fixes: c796f213a693 ("xen/trace: add multicall tracing")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Nick suggested this as well, I think it's reasonable to work around it
in this one location since this is indeed the only instance of this
warning that I see in the kernel tree across all of my builds.

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>