linux-kernel - Re: [PATCH v1 1/2] LoongArch: Define barrier_before

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Message-ID: <CAAhV-H6Mh3didzHnKJiEsMP4yVgggnBBuDS1U6yDzEvdVEbGRg@mail.gmail.com>
Date: Wed, 21 Aug 2024 15:37:49 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>, Jinyang He <hejinyang@...ngson.cn>
Cc: Arnd Bergmann <arnd@...db.de>, loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] LoongArch: Define barrier_before_unreachable() as empty

Hi, Arnd and Jinyang,

On Tue, Aug 20, 2024 at 8:37 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> When building kernel with -Os (CONFIG_CC_OPTIMIZE_FOR_SIZE=y) rather
> than the default -O2 (CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE is not set),
> there exist many objtool warnings "unreachable instruction", this is
> because objtool couldn't find a code path to reach the instruction,
> objdump shows that the unreachable instruction is related with BUG()
> or unreachable().
>
> In include/linux/compiler-gcc.h, there is an empty inline assembly
>
>   #define barrier_before_unreachable() asm volatile("")
>
> in the definition of unreachable() to work around GCC 7.0 after the
> commit 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()") in
> the year of 2018, the unreachable instruction is generated by it.
>
> As far as I can see, this workaround is to fix warnings of frame sizes,
> the default CONFIG_FRAME_WARN is 2048 on 64 bit system now, it seems
> that there are no -Wframe-larger-than warnings in the current kernel
> without this workaround.
>
> I am not sure whether the GCC bug has been fixed, I can not find the
> fixup in the link https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
> and in the GCC repo. So I am not sure whether it is time and proper
> to remove this workaround in the common header totally, just remove
> it in the arch specified header when compiling kernel with a newer
> GCC version (for example GCC 12.1 or higher on LoongArch) at least.
What's your opinion? From my point of view, this GCC bug hasn't been
fixed. So there may still be potential problems.

Huacai

>
> For example:
> arch/loongarch/kernel/cpu-probe.o: warning: objtool: cpu_probe+0x664: unreachable instruction
>
> objdump -M no-aliases -D arch/loongarch/kernel/cpu-probe.o
>
> Without this patch:
>
> 0000000000000000 <cpu_probe>:
>  ...
>  65c:   44000d80        bnez            $t0, 12 # 668 <.L345^B1+0x4>
>
> 0000000000000660 <.L10001^B2>:
>  660:   002a0001        break           0x1
>
> 0000000000000664 <.L345^B1>:
>  664:   53ff63ff        b               -160    # 5c4 <.L344^B1>
>  668:   0280040c        addi.w          $t0, $zero, 1
>  66c:   00006d8d        cpucfg          $t1, $t0
>
> With this patch:
>
> 0000000000000000 <cpu_probe>:
>  ...
>  65c:   44000980        bnez            $t0, 8  # 664 <.L345^B1>
>
> 0000000000000660 <.L10001^B2>:
>  660:   002a0001        break           0x1
>
> 0000000000000664 <.L345^B1>:
>  664:   0280040c        addi.w          $t0, $zero, 1
>  668:   00006d8d        cpucfg          $t1, $t0
>
> Cc: stable@...r.kernel.org # 6.9+
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202407221208.6SSBeN9H-lkp@intel.com/
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  arch/loongarch/Kconfig                |  1 +
>  arch/loongarch/include/asm/compiler.h | 13 +++++++++++++
>  2 files changed, 14 insertions(+)
>  create mode 100644 arch/loongarch/include/asm/compiler.h
>
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index 70f169210b52..3af0da76d103 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -102,6 +102,7 @@ config LOONGARCH
>         select GPIOLIB
>         select HAS_IOPORT
>         select HAVE_ARCH_AUDITSYSCALL
> +       select HAVE_ARCH_COMPILER_H
>         select HAVE_ARCH_JUMP_LABEL
>         select HAVE_ARCH_JUMP_LABEL_RELATIVE
>         select HAVE_ARCH_KASAN
> diff --git a/arch/loongarch/include/asm/compiler.h b/arch/loongarch/include/asm/compiler.h
> new file mode 100644
> index 000000000000..4f6adb8d533e
> --- /dev/null
> +++ b/arch/loongarch/include/asm/compiler.h
> @@ -0,0 +1,13 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2024 Loongson Technology Corporation Limited
> + */
> +#ifndef _ASM_COMPILER_H
> +#define _ASM_COMPILER_H
> +
> +#ifdef barrier_before_unreachable
> +#undef barrier_before_unreachable
> +#define barrier_before_unreachable() do { } while (0)
> +#endif
> +
> +#endif /* _ASM_COMPILER_H */
> --
> 2.42.0
>
>