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Date: Thu, 27 Nov 2014 17:42:41 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 1/6] bitmap: add atomic set functions
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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:
> +void bitmap_set_atomic(unsigned long *map, long start, long nr)
> +{
> +    unsigned long *p = map + BIT_WORD(start);
> +    const long size = start + nr;
> +    int bits_to_set = BITS_PER_LONG - (start % BITS_PER_LONG);
> +    unsigned long mask_to_set = BITMAP_FIRST_WORD_MASK(start);
> +
> +    while (nr - bits_to_set >= 0) {
> +        atomic_or(p, mask_to_set);

atomic_or is unnecessary while mask_to_set is ~0UL.  I think not even a
smp_wmb() is necessary.

Paolo

> +        nr -= bits_to_set;
> +        bits_to_set = BITS_PER_LONG;
> +        mask_to_set = ~0UL;
> +        p++;
> +    }
> +    if (nr) {
> +        mask_to_set &= BITMAP_LAST_WORD_MASK(size);
> +        atomic_or(p, mask_to_set);
> +    }
> +}
> +