Re: [PATCH v3 01/12] compiler/gcc: Emit build-time warning for GCC prior to version 4.8

From: Masahiro Yamada <masahiroy@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	"Cc: Android Kernel" <kernel-team@android.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Peter Oberparleiter <oberpar@linux.ibm.com>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v3 01/12] compiler/gcc: Emit build-time warning for GCC prior to version 4.8
Date: Thu, 16 Apr 2020 02:20:07 +0900	[thread overview]
Message-ID: <CAK7LNATMDkey2ckTfuCysVPVUAbZa5C8oBBqWtE1ura+rnCbKg@mail.gmail.com> (raw)
In-Reply-To: <20200415165218.20251-2-will@kernel.org>

On Thu, Apr 16, 2020 at 1:52 AM Will Deacon <will@kernel.org> wrote:
>
> Prior to version 4.8, GCC may miscompile READ_ONCE() by erroneously
> discarding the 'volatile' qualifier:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145
>
> We've been working around this using some nasty hacks which make
> READ_ONCE() both horribly complicated and also prevent us from enforcing
> that it is only used on scalar types. Since GCC 4.8 is pretty old for
> kernel builds now, emit a warning if we detect it during the build.

This patch is unneeded since you will remove GCC 4.8 support
in 11/12.

Please move 11/12 to the head of this series,
and remove all noise changes.

Thanks.

> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  init/Kconfig            | 4 ++--
>  scripts/Kconfig.include | 9 +++++++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 9e22ee8fbd75..816b8b4a5e9e 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -9,11 +9,11 @@ config DEFCONFIG_LIST
>         default "arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)"
>
>  config CC_IS_GCC
> -       def_bool $(success,$(CC) --version | head -n 1 | grep -q gcc)
> +       def_bool $(cc-is-gcc)
>
>  config GCC_VERSION
>         int
> -       default $(shell,$(srctree)/scripts/gcc-version.sh $(CC)) if CC_IS_GCC
> +       default $(gcc-version) if CC_IS_GCC
>         default 0
>
>  config LD_VERSION
> diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
> index c264da2b9b30..5261e9d6b50b 100644
> --- a/scripts/Kconfig.include
> +++ b/scripts/Kconfig.include
> @@ -54,3 +54,12 @@ $(error-if,$(success, $(LD) -v | grep -q gold), gold linker '$(LD)' not supporte
>  cc-option-bit = $(if-success,$(CC) -Werror $(1) -E -x c /dev/null -o /dev/null,$(1))
>  m32-flag := $(cc-option-bit,-m32)
>  m64-flag := $(cc-option-bit,-m64)
> +
> +# gcc version including patch level
> +gcc-version := $(shell,$(srctree)/scripts/gcc-version.sh $(CC))
> +
> +# Return y if the compiler is GCC, n otherwise
> +cc-is-gcc := $(success,$(CC) --version | head -n 1 | grep -q gcc)
> +
> +# Warn if the compiler is GCC prior to 4.8
> +$(warning-if,$(if-success,[ $(gcc-version) -lt 40800 ],$(cc-is-gcc),n),"Your compiler is old and may miscompile the kernel due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 - please upgrade it.")
> --
> 2.26.0.110.g2183baf09c-goog
>

-- 
Best Regards
Masahiro Yamada