linux-kernel - Re: [PATCH] riscv/barrier: Define __smp_{store_release,load

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Date:   Tue, 27 Feb 2018 10:21:43 -0800 (PST)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     parri.andrea@...il.com, Daniel Lustig <dlustig@...dia.com>
CC:     albert@...ive.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, parri.andrea@...il.com
Subject:     Re: [PATCH] riscv/barrier: Define __smp_{store_release,load_acquire}

On Mon, 26 Feb 2018 18:24:11 PST (-0800), parri.andrea@...il.com wrote:
> Introduce __smp_{store_release,load_acquire}, and rely on the generic
> definitions for smp_{store_release,load_acquire}. This avoids the use
> of full ("rw,rw") fences on SMP.
>
> Signed-off-by: Andrea Parri <parri.andrea@...il.com>
> ---
>  arch/riscv/include/asm/barrier.h | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/arch/riscv/include/asm/barrier.h b/arch/riscv/include/asm/barrier.h
> index 5510366d169ae..d4628e4b3a5ea 100644
> --- a/arch/riscv/include/asm/barrier.h
> +++ b/arch/riscv/include/asm/barrier.h
> @@ -38,6 +38,21 @@
>  #define __smp_rmb()	RISCV_FENCE(r,r)
>  #define __smp_wmb()	RISCV_FENCE(w,w)
>
> +#define __smp_store_release(p, v)					\
> +do {									\
> +	compiletime_assert_atomic_type(*p);				\
> +	RISCV_FENCE(rw,w);						\
> +	WRITE_ONCE(*p, v);						\
> +} while (0)
> +
> +#define __smp_load_acquire(p)						\
> +({									\
> +	typeof(*p) ___p1 = READ_ONCE(*p);				\
> +	compiletime_assert_atomic_type(*p);				\
> +	RISCV_FENCE(r,rw);						\
> +	___p1;								\
> +})
> +
>  /*
>   * This is a very specific barrier: it's currently only used in two places in
>   * the kernel, both in the scheduler.  See include/linux/spinlock.h for the two

I'm adding Daniel just in case I misunderstood what's going on here, but these 
look good to me.  As this is a non-trivial memory model change I'm going to let 
it bake in linux-next for a bit just so it gets some visibility.

Thanks!