linux-kernel - Re: [PATCH 7/7] microblaze: Do atomic operations by using exclusive ops

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Date:   Wed, 12 Feb 2020 16:55:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michal Simek <michal.simek@...inx.com>
Cc:     linux-kernel@...r.kernel.org, monstr@...str.eu, git@...inx.com,
        arnd@...db.de,
        Stefan Asserhall load and store 
        <stefan.asserhall@...inx.com>, Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 7/7] microblaze: Do atomic operations by using exclusive
 ops

On Wed, Feb 12, 2020 at 04:42:29PM +0100, Michal Simek wrote:
> From: Stefan Asserhall load and store <stefan.asserhall@...inx.com>
> 
> Implement SMP aware atomic operations.
> 
> Signed-off-by: Stefan Asserhall <stefan.asserhall@...inx.com>
> Signed-off-by: Michal Simek <michal.simek@...inx.com>
> ---
> 
>  arch/microblaze/include/asm/atomic.h | 265 +++++++++++++++++++++++++--
>  1 file changed, 253 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/microblaze/include/asm/atomic.h b/arch/microblaze/include/asm/atomic.h
> index 41e9aff23a62..522d704fad63 100644
> --- a/arch/microblaze/include/asm/atomic.h
> +++ b/arch/microblaze/include/asm/atomic.h
> @@ -1,28 +1,269 @@
>  /* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2013-2020 Xilinx, Inc.
> + */
> +
>  #ifndef _ASM_MICROBLAZE_ATOMIC_H
>  #define _ASM_MICROBLAZE_ATOMIC_H
>  
> +#include <linux/types.h>
>  #include <asm/cmpxchg.h>
> -#include <asm-generic/atomic.h>
> -#include <asm-generic/atomic64.h>
> +
> +#define ATOMIC_INIT(i)	{ (i) }
> +
> +#define atomic_read(v)	READ_ONCE((v)->counter)
> +
> +static inline void atomic_set(atomic_t *v, int i)
> +{
> +	int result, tmp;
> +
> +	__asm__ __volatile__ (
> +		/* load conditional address in %2 to %0 */
> +		"1:	lwx	%0, %2, r0;\n"
> +		/* attempt store */
> +		"	swx	%3, %2, r0;\n"
> +		/* checking msr carry flag */
> +		"	addic	%1, r0, 0;\n"
> +		/* store failed (MSR[C] set)? try again */
> +		"	bnei	%1, 1b;\n"
> +		/* Outputs: result value */
> +		: "=&r" (result), "=&r" (tmp)
> +		/* Inputs: counter address */
> +		: "r" (&v->counter), "r" (i)
> +		: "cc", "memory"
> +	);
> +}
> +#define atomic_set	atomic_set

Uuuuhh.. *what* ?!?

Are you telling me your LL/SC implementation is so bugger that
atomic_set() being a WRITE_ONCE() does not in fact work?