linux-kernel - Re: [PATCH v2 1/2] compiler-gcc.h: add gnu

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Date:   Tue, 05 Jun 2018 10:23:40 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        akpm@...ux-foundation.org, ard.biesheuvel@...aro.org,
        aryabinin@...tuozzo.com, akataria@...are.com,
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        hpa@...or.com, jan.kiszka@...mens.com,
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        jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com,
        mingo@...hat.com, mjg59@...gle.com, mka@...omium.org,
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        thomas.lendacky@....com, tweek@...gle.com
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
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        rientjes@...gle.com
Subject: Re: [PATCH v2 1/2] compiler-gcc.h: add gnu_inline to all inline
 declarations

On Tue, 2018-06-05 at 10:05 -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 +72,24 @@
>   * -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__ __attribute__((always_inline, unused, gnu_inline)) notrace
> +#define __inline \
> +	__inline __attribute__((always_inline, unused, gnu_inline)) notrace

Perhaps these are simpler as

#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__	__attribute__((unused, gnu_inline)) notrace
> +#define __inline __inline	__attribute__((unused, gnu_inline)) notrace
>  #endif

And only set once along with:

>  #define __always_inline	inline __attribute__((always_inline))

And perhaps this __always_inline should be updated
with gnu_inline as well