linux-kernel - Re: [PATCH v3 1/3] compiler-gcc.h: add gnu

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Date:   Thu, 07 Jun 2018 11:43:58 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        akpm@...ux-foundation.org, hpa@...or.com, mingo@...hat.com,
        tglx@...utronix.de
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, virtualization@...ts.linux-foundation.org,
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        sedat.dilek@...il.com, tstellar@...hat.com, keescook@...gle.com,
        yamada.masahiro@...ionext.com, michal.lkml@...kovi.net,
        linux-kbuild@...r.kernel.org, geert@...ux-m68k.org,
        will.deacon@....com, mawilcox@...rosoft.com, arnd@...db.de,
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        thomas.lendacky@....com, tweek@...gle.com, mjg59@...gle.com
Subject: Re: [PATCH v3 1/3] compiler-gcc.h: add gnu_inline to all inline
 declarations

On Thu, 2018-06-07 at 11:32 -0700, Nick Desaulniers wrote:
> Functions marked extern inline do not emit an externally visible
> function when the gnu89 C standard is used. Some KBUILD Makefiles
> overwrite KBUILD_CFLAGS. This is an issue for GCC 5.1+ users as without
> an explicit C standard specified, the default is gnu11. Since c99, the
> semantics of extern inline have changed such that an externally visible
> function is always emitted. This can lead to multiple definition errors
> of extern inline functions at link time of compilation units whose build
> files have removed an explicit C standard compiler flag for users of GCC
> 5.1+ or Clang.
[]
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
[]
> @@ -72,17 +72,22 @@
>   * -Wunused-function.  This turns out to avoid the need for complex #ifdef
>   * directives.  Suppress the warning in clang as well by using "unused"
>   * function attribute, which is redundant but not harmful for gcc.
> + * Prefer gnu_inline, so that extern inline functions do not emit an
> + * externally visible function. This makes extern inline behave as per gnu89
> + * semantics rather than c99. This prevents multiple symbol definition errors
> + * of extern inline functions at link time.
>   */
>  #if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||		\
>      !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
> -#define inline inline		__attribute__((always_inline,unused)) notrace
> -#define __inline__ __inline__	__attribute__((always_inline,unused)) notrace
> -#define __inline __inline	__attribute__((always_inline,unused)) notrace
> +#define inline \
> +	inline __attribute__((always_inline, unused, gnu_inline)) notrace
> +#define __inline__ inline
> +#define __inline inline
>  #else
>  /* A lot of inline functions can cause havoc with function tracing */
> -#define inline inline		__attribute__((unused)) notrace
> -#define __inline__ __inline__	__attribute__((unused)) notrace
> -#define __inline __inline	__attribute__((unused)) notrace
> +#define inline inline		__attribute__((unused, gnu_inline)) notrace
> +#define __inline__ inline
> +#define __inline inline
>  #endif

Perhaps move these last 2 defines below the #endif
as they are common to both cases

#if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||               \
    !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
#define inline inline __attribute__((always_inline, unused, gnu_inline)) notrace
#else
/* A lot of inline functions can cause havoc with function tracing */
#define inline inline __attribute__((unused, gnu_inline)) notrace
#endif

#define __inline__	inline
#define __inline	inline
#define __always_inline	inline __attribute__((always_inline))