Re: [Qemu-devel] [PATCH v3 23/29] bitmap: add atomic set functions

[Fam Zheng <famz@redhat.com>]

On Tue, 05/26 18:54, Paolo Bonzini wrote:
> From: Stefan Hajnoczi <stefanha@redhat.com>
> 
> 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 <stefanha@redhat.com>
> Message-Id: <1417519399-3166-2-git-send-email-stefanha@redhat.com>
> [Avoid barrier in the single word case, use full barrier instead of write.
>  - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  include/qemu/bitmap.h |  2 ++
>  include/qemu/bitops.h | 14 ++++++++++++++
>  util/bitmap.c         | 38 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 54 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

The tab/space mix is not consistent. Fixing can be on top.

>   * 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 8abdcf9..8164225 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);
> +}

Where is this function used?

Fam