linux-kernel - Re: [PATCH 2/3] arm64: add __READ_ONCE

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Date: Tue, 2 Apr 2024 17:48:46 +0100
From: Mark Rutland <mark.rutland@....com>
To: Haris Okanovic <harisokn@...zon.com>
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-assembly@...r.kernel.org, peterz@...radead.org
Subject: Re: [PATCH 2/3] arm64: add __READ_ONCE_EX()

On Mon, Apr 01, 2024 at 08:47:05PM -0500, Haris Okanovic wrote:
> Perform an exclusive load, which atomically loads a word and arms the
> execusive monitor to enable wfe() polling of an address.
> 
> Adding this macro in preparation for an arm64 cpuidle driver which
> supports a wfe() based polling state.
> 
> https://developer.arm.com/documentation/dht0008/a/arm-synchronization-primitives/exclusive-accesses/exclusive-monitors
> 
> Signed-off-by: Haris Okanovic <harisokn@...zon.com>
> ---
>  arch/arm64/include/asm/readex.h | 46 +++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
>  create mode 100644 arch/arm64/include/asm/readex.h
> 
> diff --git a/arch/arm64/include/asm/readex.h b/arch/arm64/include/asm/readex.h
> new file mode 100644
> index 000000000000..51963c3107e1
> --- /dev/null
> +++ b/arch/arm64/include/asm/readex.h
> @@ -0,0 +1,46 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Based on arch/arm64/include/asm/rwonce.h
> + *
> + * Copyright (C) 2020 Google LLC.
> + * Copyright (C) 2024 Amazon.com, Inc. or its affiliates.
> + */
> +
> +#ifndef __ASM_READEX_H
> +#define __ASM_READEX_H
> +
> +#define __LOAD_EX(sfx, regs...) "ldaxr" #sfx "\t" #regs
> +
> +#define __READ_ONCE_EX(x)						\
> +({									\
> +	typeof(&(x)) __x = &(x);					\
> +	int atomic = 1;							\
> +	union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u;	\
> +	switch (sizeof(x)) {						\
> +	case 1:								\
> +		asm volatile(__LOAD_EX(b, %w0, %1)			\
> +			: "=r" (*(__u8 *)__u.__c)			\
> +			: "Q" (*__x) : "memory");			\
> +		break;							\
> +	case 2:								\
> +		asm volatile(__LOAD_EX(h, %w0, %1)			\
> +			: "=r" (*(__u16 *)__u.__c)			\
> +			: "Q" (*__x) : "memory");			\
> +		break;							\
> +	case 4:								\
> +		asm volatile(__LOAD_EX(, %w0, %1)			\
> +			: "=r" (*(__u32 *)__u.__c)			\
> +			: "Q" (*__x) : "memory");			\
> +		break;							\
> +	case 8:								\
> +		asm volatile(__LOAD_EX(, %0, %1)			\
> +			: "=r" (*(__u64 *)__u.__c)			\
> +			: "Q" (*__x) : "memory");			\
> +		break;							\
> +	default:							\
> +		atomic = 0;						\
> +	}								\
> +	atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(__x))__x);\
> +})

Why can't you use the existing smp_cond_load_relaxed() or
smp_cond_load_acquire()?

I don't believe this is necessary.

Mark.