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Re: [Qemu-devel] [PATCH 23/29] bitmap: add atomic set functions
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [PATCH 23/29] bitmap: add atomic set functions |
Date: |
Tue, 26 May 2015 20:36:07 +0800 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Tue, 05/26 19:54, Fam Zheng wrote:
> On Mon, 04/27 18:28, Paolo Bonzini wrote:
> > From: Stefan Hajnoczi <address@hidden>
> >
> > Use atomic_or() for atomic bitmaps where several threads may set bits at
> > the same time. This avoids the race condition between threads loading
> > an element, bitwise ORing, and then storing the element.
> >
> > When setting all bits in a word we can avoid atomic ops and instead just
> > use an smp_mb() at the end.
> >
> > Most bitmap users don't need atomicity so introduce new functions.
> >
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > Message-Id: <address@hidden>
> > [Avoid barrier in the single word case, use full barrier instead of write.
> > - Paolo]
> > Signed-off-by: Paolo Bonzini <address@hidden>
> > ---
> > include/qemu/bitmap.h | 2 ++
> > include/qemu/bitops.h | 14 ++++++++++++++
> > util/bitmap.c | 37 +++++++++++++++++++++++++++++++++++++
> > 3 files changed, 53 insertions(+)
> >
> > diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h
> > index f0273c9..3e0a4f3 100644
> > --- a/include/qemu/bitmap.h
> > +++ b/include/qemu/bitmap.h
> > @@ -39,6 +39,7 @@
> > * bitmap_empty(src, nbits) Are all bits zero in
> > *src?
> > * bitmap_full(src, nbits) Are all bits set in *src?
> > * 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_find_next_zero_area(buf, len, pos, n, mask) Find bit free
> > area
> > */
> > @@ -226,6 +227,7 @@ 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);
> > unsigned long bitmap_find_next_zero_area(unsigned long *map,
> > unsigned long size,
> > diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
> > index 90ca8df..95f98fb 100644
> > --- a/include/qemu/bitops.h
> > +++ b/include/qemu/bitops.h
> > @@ -16,6 +16,7 @@
> > #include <assert.h>
> >
> > #include "host-utils.h"
> > +#include "atomic.h"
> >
> > #define BITS_PER_BYTE CHAR_BIT
> > #define BITS_PER_LONG (sizeof (unsigned long) * BITS_PER_BYTE)
> > @@ -39,6 +40,19 @@ static inline void set_bit(long nr, unsigned long *addr)
> > }
> >
> > /**
> > + * set_bit_atomic - Set a bit in memory atomically
> > + * @nr: the bit to set
> > + * @addr: the address to start counting from
> > + */
> > +static inline void set_bit_atomic(long nr, unsigned long *addr)
> > +{
> > + unsigned long mask = BIT_MASK(nr);
> > + unsigned long *p = addr + BIT_WORD(nr);
> > +
> > + atomic_or(p, mask);
> > +}
> > +
> > +/**
> > * clear_bit - Clears a bit in memory
> > * @nr: Bit to clear
> > * @addr: Address to start counting from
> > diff --git a/util/bitmap.c b/util/bitmap.c
> > index 9c6bb52..6838d49 100644
> > --- a/util/bitmap.c
> > +++ b/util/bitmap.c
> > @@ -11,6 +11,7 @@
> >
> > #include "qemu/bitops.h"
> > #include "qemu/bitmap.h"
> > +#include "qemu/atomic.h"
> >
> > /*
> > * bitmaps provide an array of bits, implemented using an an
> > @@ -177,6 +178,42 @@ void bitmap_set(unsigned long *map, long start, long
> > nr)
> > }
> > }
> >
> > +void bitmap_set_atomic(unsigned long *map, long start, long nr)
> > +{
> > + unsigned long *p = map + BIT_WORD(start);
> > + const long size = start + nr;
>
> s/size/end/ ?
>
> Otherwise,
>
> Reviewed-by: Fam Zheng <address@hidden>
NACK
It's broken the same way as bitmap_test_and_clear_atomic. See my reply to patch
24.
Fam
>
> > + int bits_to_set = BITS_PER_LONG - (start % BITS_PER_LONG);
> > + unsigned long mask_to_set = BITMAP_FIRST_WORD_MASK(start);
> > +
> > + /* First word */
> > + if (nr - bits_to_set > 0) {
> > + atomic_or(p, mask_to_set);
> > + nr -= bits_to_set;
> > + bits_to_set = BITS_PER_LONG;
> > + mask_to_set = ~0UL;
> > + p++;
> > + }
> > +
> > + /* Full words */
> > + while (nr - bits_to_set >= 0) {
> > + *p = ~0UL;
> > + nr -= bits_to_set;
> > + mask_to_set = ~0UL;
> > + p++;
> > + }
>
> For full words, will memset be faster by any chance?
>
> > +
> > + /* Last word */
> > + if (nr) {
> > + mask_to_set &= BITMAP_LAST_WORD_MASK(size);
> > + atomic_or(p, mask_to_set);
> > + } else {
> > + /* If we avoided the full barrier in atomic_or(), issue a
> > + * barrier to account for the assignments in the while loop.
> > + */
> > + smp_mb();
> > + }
> > +}
> > +
> > void bitmap_clear(unsigned long *map, long start, long nr)
> > {
> > unsigned long *p = map + BIT_WORD(start);
> > --
> > 1.8.3.1
> >
> >
>