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From: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <7ec6ba72-fee7-52db-a73a-be1720a06383@linaro.org>
Date: Thu, 26 Apr 2018 14:14:37 -1000
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Subject: Re: [Qemu-devel] [PATCH v8 22/35] RISC-V: Use atomic_cmpxchg to
 update PLIC bitmaps
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To: Michael Clark <mjc@sifive.com>, qemu-devel@nongnu.org
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>, Bastian Koppelmann <kbastian@mail.uni-paderborn.de>, Palmer Dabbelt <palmer@sifive.com>, Alistair Francis <Alistair.Francis@wdc.com>, patches@groups.riscv.org

On 04/25/2018 01:45 PM, Michael Clark wrote:
> +    uint32_t old, new;
> +    do {
> +        old = atomic_read(&plic->pending[word]);
> +        new = (old & ~(1 << (irq & 31))) | (-!!pending & (1 << (irq & 31)));
> +    } while (atomic_cmpxchg(&plic->pending[word], old, new) != old);

I prefer

  uint32_t old, new, cmp = atomic_read(...);
  do {
    old = cmp;
    new = ...;
    cmp = atomic_cmpxchg(...);
  } while (old != cmp);

to avoid an extra load, should we loop.

That said, what you have is not wrong.  So,
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~