linux-kernel - Re: [PATCH] csky: atomic: Add custom atomic.h implementation

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Date:   Mon, 11 Apr 2022 10:40:46 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     guoren@...nel.org
Cc:     arnd@...db.de, peterz@...radead.org, linux-csky@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH] csky: atomic: Add custom atomic.h implementation

On Wed, Apr 06, 2022 at 08:54:36PM +0800, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
> 
> The generic atomic.h used cmpxchg to implement the atomic
> operations, it will cause daul loop to reduce the forward
> guarantee. The patch implement csky custom atomic operations with
> ldex/stex instructions for the best performance.
> 
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> ---
>  arch/csky/include/asm/atomic.h | 251 +++++++++++++++++++++++++++++++++
>  1 file changed, 251 insertions(+)
>  create mode 100644 arch/csky/include/asm/atomic.h

> +static __always_inline							\
> +int arch_atomic_fetch_##op(int i, atomic_t *v)				\
> +{									\
> +	register int ret, tmp;						\
> +	__asm__ __volatile__ (						\
> +	"1:	ldex.w		%0, (%3) \n"				\
> +	ACQUIRE_FENCE							\
> +	"	mov		%1, %0   \n"				\
> +	"	" #op "		%0, %2   \n"				\
> +	RELEASE_FENCE							\
> +	"	stex.w		%0, (%3) \n"				\
> +	"	bez		%0, 1b   \n"				\
> +		: "=&r" (tmp), "=&r" (ret)				\
> +		: "r" (I), "r"(&v->counter) 				\
> +		: "memory");						\
> +	return ret;							\
> +}

I believe this suffers the problem described in:

  8e86f0b409a44193 ("arm64: atomics: fix use of acquire + release for full barrier semantics")

... and does not provide FULL ordering semantics.

Thanks,
Mark.