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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 1 Jun 2021 11:23:03 -0700
Message-ID: <CAKwvOd=Z60pm4rZw5yhpq-vCKb_xqxKa9xtgsCoknzD4VNj2wQ@mail.gmail.com>
Subject: Re: [PATCH] powerpc/barrier: Avoid collision with clang's __lwsync macro
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        "# 3.4.x" <stable@vger.kernel.org>
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On Fri, May 28, 2021 at 11:29 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> A change in clang 13 results in the __lwsync macro being defined as
> __builtin_ppc_lwsync, which emits 'lwsync' or 'msync' depending on what
> the target supports.

Indeed,
$ clang --target=powerpc64le-linux-gnu -mcpu=e500 -m32
for example.

Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> This breaks the build because of -Werror in
> arch/powerpc, along with thousands of warnings:
>
>  In file included from arch/powerpc/kernel/pmc.c:12:
>  In file included from include/linux/bug.h:5:
>  In file included from arch/powerpc/include/asm/bug.h:109:
>  In file included from include/asm-generic/bug.h:20:
>  In file included from include/linux/kernel.h:12:
>  In file included from include/linux/bitops.h:32:
>  In file included from arch/powerpc/include/asm/bitops.h:62:
>  arch/powerpc/include/asm/barrier.h:49:9: error: '__lwsync' macro redefined [-Werror,-Wmacro-redefined]
>  #define __lwsync()      __asm__ __volatile__ (stringify_in_c(LWSYNC) : : :"memory")
>         ^
>  <built-in>:308:9: note: previous definition is here
>  #define __lwsync __builtin_ppc_lwsync
>         ^
>  1 error generated.
>
> Undefine this macro so that the runtime patching introduced by
> commit 2d1b2027626d ("powerpc: Fixup lwsync at runtime") continues to
> work properly with clang and the build no longer breaks.
>
> Cc: stable@vger.kernel.org
> Link: https://github.com/ClangBuiltLinux/linux/issues/1386
> Link: https://github.com/llvm/llvm-project/commit/62b5df7fe2b3fda1772befeda15598fbef96a614
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  arch/powerpc/include/asm/barrier.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index 7ae29cfb06c0..f0e687236484 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -46,6 +46,8 @@
>  #    define SMPWMB      eieio
>  #endif
>
> +/* clang defines this macro for a builtin, which will not work with runtime patching */
> +#undef __lwsync
>  #define __lwsync()     __asm__ __volatile__ (stringify_in_c(LWSYNC) : : :"memory")
>  #define dma_rmb()      __lwsync()
>  #define dma_wmb()      __asm__ __volatile__ (stringify_in_c(SMPWMB) : : :"memory")
>
> base-commit: 97e5bf604b7a0d6e1b3e00fe31d5fd4b9bffeaae
> --
> 2.32.0.rc0
>


-- 
Thanks,
~Nick Desaulniers