[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 24/29] bitmap: add atomic test and clear
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [PATCH 24/29] bitmap: add atomic test and clear |
Date: |
Tue, 26 May 2015 20:32:32 +0800 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Tue, 05/26 20:27, Fam Zheng wrote:
> On Mon, 04/27 18:28, Paolo Bonzini wrote:
> > From: Stefan Hajnoczi <address@hidden>
> >
> > 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 <address@hidden>
> > Message-Id: <address@hidden>
> > [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 <address@hidden>
> > ---
> > 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
>