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

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Date: Mon, 8 Apr 2024 14:51:13 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Haris Okanovic' <harisokn@...zon.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-pm@...r.kernel.org"
	<linux-pm@...r.kernel.org>, "linux-assembly@...r.kernel.org"
	<linux-assembly@...r.kernel.org>
CC: "peterz@...radead.org" <peterz@...radead.org>
Subject: RE: [PATCH 2/3] arm64: add __READ_ONCE_EX()

From: Haris Okanovic
> Sent: 02 April 2024 02:47
> 
> 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);\

I'm pretty sure that doesn't work the way you expect.
The ?: operator promotes 'unsigned char' to  'int'.
So you can fall foul of signedness tests (eg in min()).
It also isn't going to work for non-scalers.

Replacing the ?: with __builtin_choose_expr() may help.

(This is probably a bug in the code you copied.)

	David

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