Re: [Qemu-devel] [PATCH 24/29] bitmap: add atomic test and clear

[Fam Zheng <famz@redhat.com>]

On Tue, 05/26 20:27, Fam Zheng wrote:
> On Mon, 04/27 18:28, Paolo Bonzini wrote:
> > From: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > 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>
> > Message-Id: <1417519399-3166-3-git-send-email-stefanha@redhat.com>
> > [Test before xchg; then a full barrier is needed at the end just like
> >  in the previous patch.  The barrier can be avoided if we did at least
> >  one xchg.  - Paolo]
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >  include/qemu/bitmap.h |  2 ++
> >  util/bitmap.c         | 44 ++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 46 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 6838d49..d3e088d 100644
> > --- a/util/bitmap.c
> > +++ b/util/bitmap.c
> > @@ -234,6 +234,50 @@ 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++;
> > +    }
> > +
> 
> It's broken if the clear range is the last part of first word:
> 
> map =        0xFFFFFFFFFFFFFFFF, ...
> 
> bitmap_test_and_clear_atomic(map, 60, 4):
> 
> Expected result:
> 
>              0xFFFFFFFFFFFFFFFF, ...

Sorry, I meant 0xFFFFFFFFFFFFFFF (the highest 4 bits being cleared).

> 
> Actual result:
> 
>    0x0
> 
> Fam
>