# Re: \[PATCH 18/18\] arm64: lto: Strengthen READ\_ONCE() to acquire when CLANG\_LTO=y

- From: Dave P Martin \<dave.martin@arm.com\>
- Date: 2020-07-01 17:07:37 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2020/07/6022150
- Thread: https://ratatoskr.run/linux-arm-kernel/2020/06/6020640/t.md

```text
On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote:
> When building with LTO, there is an increased risk of the compiler
> converting an address dependency headed by a READ_ONCE() invocation
> into a control dependency and consequently allowing for harmful
> reordering by the CPU.
> 
> Ensure that such transformations are harmless by overriding the generic
> READ_ONCE() definition with one that provides acquire semantics when
> building with LTO.
> 
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/include/asm/rwonce.h   | 63 +++++++++++++++++++++++++++++++
>  arch/arm64/kernel/vdso/Makefile   |  2 +-
>  arch/arm64/kernel/vdso32/Makefile |  2 +-
>  3 files changed, 65 insertions(+), 2 deletions(-)
>  create mode 100644 arch/arm64/include/asm/rwonce.h
> 
> diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> new file mode 100644
> index 000000000000..515e360b01a1
> --- /dev/null
> +++ b/arch/arm64/include/asm/rwonce.h
> @@ -0,0 +1,63 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2020 Google LLC.
> + */
> +#ifndef __ASM_RWONCE_H
> +#define __ASM_RWONCE_H
> +
> +#ifdef CONFIG_CLANG_LTO

Don't we have a generic option for LTO that's not specific to Clang.

Also, can you illustrate code that can only be unsafe with Clang LTO?

[...]

Cheers
---Dave
```