On Tue, Jun 06, 2017 at 04:00:03PM -0700, Palmer Dabbelt wrote:
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> new file mode 100644
> index 000000000000..c7ee1321ac18
> --- /dev/null
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -0,0 +1,124 @@
> +/*
> + * Copyright (C) 2014 Regents of the University of California
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation, version 2.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + */
> +
> +#ifndef _ASM_RISCV_CMPXCHG_H
> +#define _ASM_RISCV_CMPXCHG_H
> +
> +#include <linux/bug.h>
> +
> +#ifdef CONFIG_ISA_A
> +
> +#include <asm/barrier.h>
> +
> +#define __xchg(new, ptr, size) \
> +({ \
> + __typeof__(ptr) __ptr = (ptr); \
> + __typeof__(new) __new = (new); \
> + __typeof__(*(ptr)) __ret; \
> + switch (size) { \
> + case 4: \
> + __asm__ __volatile__ ( \
> + "amoswap.w %0, %2, %1" \
> + : "=r" (__ret), "+A" (*__ptr) \
> + : "r" (__new)); \
> + break; \
> + case 8: \
> + __asm__ __volatile__ ( \
> + "amoswap.d %0, %2, %1" \
> + : "=r" (__ret), "+A" (*__ptr) \
> + : "r" (__new)); \
> + break; \
> + default: \
> + BUILD_BUG(); \
> + } \
> + __ret; \
> +})
> +
> +#define xchg(ptr, x) (__xchg((x), (ptr), sizeof(*(ptr))))
our xchg() is fully ordered, and thus you need to use "amoswap.aq.rl"
> +
> +
> +/*
> + * Atomic compare and exchange. Compare OLD with MEM, if identical,
> + * store NEW in MEM. Return the initial value in MEM. Success is
> + * indicated by comparing RETURN with OLD.
> + */
> +#define __cmpxchg(ptr, old, new, size) \
> +({ \
> + __typeof__(ptr) __ptr = (ptr); \
> + __typeof__(old) __old = (old); \
> + __typeof__(new) __new = (new); \
> + __typeof__(*(ptr)) __ret; \
> + register unsigned int __rc; \
> + switch (size) { \
> + case 4: \
> + __asm__ __volatile__ ( \
> + "0:" \
> + "lr.w %0, %2\n" \
> + "bne %0, %z3, 1f\n" \
> + "sc.w %1, %z4, %2\n" \
> + "bnez %1, 0b\n" \
> + "1:" \
> + : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> + : "rJ" (__old), "rJ" (__new)); \
> + break; \
> + case 8: \
> + __asm__ __volatile__ ( \
> + "0:" \
> + "lr.d %0, %2\n" \
> + "bne %0, %z3, 1f\n" \
> + "sc.d %1, %z4, %2\n" \
> + "bnez %1, 0b\n" \
> + "1:" \
> + : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> + : "rJ" (__old), "rJ" (__new)); \
> + break; \
> + default: \
> + BUILD_BUG(); \
> + } \
> + __ret; \
> +})
> +
> +#define __cmpxchg_mb(ptr, old, new, size) \
> +({ \
> + __typeof__(*(ptr)) __ret; \
> + smp_mb(); \
> + __ret = __cmpxchg((ptr), (old), (new), (size)); \
> + smp_mb(); \
> + __ret; \
> +})
Your ISA of course, but wouldn't setting the AQ and RL bits on LR/SC be
cheaper than doing two full barriers around the thing?
Note that cmpxchg() doesn't need to provide ordering on failure.
Further note that we have:
{atomic_,}cmpxchg{_relaxed,_acquire,_release,}()
and recently:
{atomic_,}try_cmpxchg{_relaxed,_acquire,_release,}()
