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Date: Thu, 27 Nov 2014 17:43:56 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 2/6] bitmap: add atomic test and clear
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To: Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org
Cc: Peter Maydell <peter.maydell@linaro.org>, rth@redhat.com, Juan Quintela <quintela@redhat.com>



On 27/11/2014 13:29, Stefan Hajnoczi wrote:
> +bool bitmap_test_and_clear_atomic(unsigned long *map, long start, long nr)
> +{
> +    unsigned long *p = map + BIT_WORD(start);
> +    const long size = start + nr;
> +    int bits_to_clear = BITS_PER_LONG - (start % BITS_PER_LONG);
> +    unsigned long mask_to_clear = BITMAP_FIRST_WORD_MASK(start);
> +    unsigned long dirty = 0;
> +    unsigned long old_bits;
> +
> +    while (nr - bits_to_clear >= 0) {
> +        old_bits = atomic_fetch_and(p, ~mask_to_clear);
> +        dirty |= old_bits & mask_to_clear;
> +        nr -= bits_to_clear;
> +        bits_to_clear = BITS_PER_LONG;
> +        mask_to_clear = ~0UL;
> +        p++;
> +    }
> +    if (nr) {
> +        mask_to_clear &= BITMAP_LAST_WORD_MASK(size);
> +        old_bits = atomic_fetch_and(p, ~mask_to_clear);
> +        dirty |= old_bits & mask_to_clear;
> +    }
> +
> +    return dirty;
> +}

Same here; you can use atomic_xchg, which is faster because on x86
atomic_fetch_and must do a compare-and-swap loop.

Paolo