Re: [Qemu-devel] [PATCH v8 22/35] RISC-V: Use atomic_cmpxchg to update PLIC bitmaps

[Richard Henderson]

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 <address@hidden>


r~