Re: [Qemu-devel] [RFC v2 2/6] bitmap: add atomic test and clear

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/12/2014 12:23, Stefan Hajnoczi wrote:
> The new bitmap_test_and_clear_atomic() function clears a range and
> returns whether or not the bits were set.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/qemu/bitmap.h |  2 ++
>  util/bitmap.c         | 37 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h
> index 3e0a4f3..86dd9cd 100644
> --- a/include/qemu/bitmap.h
> +++ b/include/qemu/bitmap.h
> @@ -41,6 +41,7 @@
>   * bitmap_set(dst, pos, nbits)			Set specified bit area
>   * bitmap_set_atomic(dst, pos, nbits)   Set specified bit area with atomic ops
>   * bitmap_clear(dst, pos, nbits)		Clear specified bit area
> + * bitmap_test_and_clear_atomic(dst, pos, nbits)    Test and clear area
>   * bitmap_find_next_zero_area(buf, len, pos, n, mask)	Find bit free area
>   */
>  
> @@ -229,6 +230,7 @@ static inline int bitmap_intersects(const unsigned long *src1,
>  void bitmap_set(unsigned long *map, long i, long len);
>  void bitmap_set_atomic(unsigned long *map, long i, long len);
>  void bitmap_clear(unsigned long *map, long start, long nr);
> +bool bitmap_test_and_clear_atomic(unsigned long *map, long start, long nr);
>  unsigned long bitmap_find_next_zero_area(unsigned long *map,
>                                           unsigned long size,
>                                           unsigned long start,
> diff --git a/util/bitmap.c b/util/bitmap.c
> index 40db0d9..d297066 100644
> --- a/util/bitmap.c
> +++ b/util/bitmap.c
> @@ -233,6 +233,43 @@ void bitmap_clear(unsigned long *map, long start, long nr)
>      }
>  }
>  
> +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;
> +
> +    /* First word */
> +    if (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++;
> +    }
> +
> +    /* Full words */
> +    while (nr - bits_to_clear >= 0) {
> +        old_bits = atomic_xchg(p, 0);

Perhaps "if (!*p) continue;", and a smp_wmb or smp_mb at the end as in
patch 1?  That's at least what KVM does.

Paolo

> +        dirty |= old_bits;
> +        nr -= bits_to_clear;
> +        p++;
> +    }
> +
> +    /* Last word */
> +    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;
> +}
> +
>  #define ALIGN_MASK(x,mask)      (((x)+(mask))&~(mask))
>  
>  /**
>