linux-kernel - Re: [PATCH V2 11/19] csky: Atomic operations

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Date:   Thu, 5 Jul 2018 19:50:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <ren_guo@...ky.com>
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, daniel.lezcano@...aro.org,
        jason@...edaemon.net, arnd@...db.de, c-sky_gcc_upstream@...ky.com,
        gnu-csky@...tor.com, thomas.petazzoni@...tlin.com,
        wbx@...ibc-ng.org, green.hu@...il.com,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH V2 11/19] csky: Atomic operations

On Mon, Jul 02, 2018 at 01:30:14AM +0800, Guo Ren wrote:

> +#include <asm/barrier.h>
> +
> +#define __xchg(new, ptr, size)					\
> +({								\
> +	__typeof__(ptr) __ptr = (ptr);				\
> +	__typeof__(new) __new = (new);				\
> +	__typeof__(*(ptr)) __ret;				\
> +	unsigned long tmp;					\
> +	switch (size) {						\
> +	case 4:							\
> +		asm volatile (					\
> +		"1:	ldex.w		%0, (%3) \n"		\
> +		"	mov		%1, %2   \n"		\
> +		"	stex.w		%1, (%3) \n"		\
> +		"	bez		%1, 1b   \n"		\
> +			: "=&r" (__ret), "=&r" (tmp)		\
> +			: "r" (__new), "r"(__ptr)		\
> +			: "memory");				\
> +		smp_mb();					\
> +		break;						\
> +	default:						\
> +		BUILD_BUG();					\
> +	}							\
> +	__ret;							\
> +})
> +
> +#define xchg(ptr, x)	(__xchg((x), (ptr), sizeof(*(ptr))))
> +
> +#define __cmpxchg(ptr, old, new, size)				\
> +({								\
> +	__typeof__(ptr) __ptr = (ptr);				\
> +	__typeof__(new) __new = (new);				\
> +	__typeof__(new) __tmp;					\
> +	__typeof__(old) __old = (old);				\
> +	__typeof__(*(ptr)) __ret;				\
> +	switch (size) {						\
> +	case 4:							\
> +		asm volatile (					\
> +		"1:	ldex.w		%0, (%3) \n"		\
> +		"	cmpne		%0, %4   \n"		\
> +		"	bt		2f       \n"		\
> +		"	mov		%1, %2   \n"		\
> +		"	stex.w		%1, (%3) \n"		\
> +		"	bez		%1, 1b   \n"		\
> +		"2:				 \n"		\
> +			: "=&r" (__ret), "=&r" (__tmp)		\
> +			: "r" (__new), "r"(__ptr), "r"(__old)	\
> +			: "memory");				\
> +		smp_mb();					\
> +		break;						\
> +	default:						\
> +		BUILD_BUG();					\
> +	}							\
> +	__ret;							\
> +})
> +
> +#define cmpxchg(ptr, o, n) \
> +	(__cmpxchg((ptr), (o), (n), sizeof(*(ptr))))

What's the memory ordering rules for your LDEX/STEX ?

The mandated semantics for xchg() / cmpxchg() is an effective smp_mb()
before _and_ after.

The above implementation suggests LDEX implies a SYNC.IS, is this
correct?