Re: [Qemu-devel] [PATCH 28/29] memory: make cpu_physical_memory_sync_dirty_bitmap() fully atomic

[Fam Zheng <famz@redhat.com>]

On Mon, 04/27 18:28, Paolo Bonzini wrote:
> From: Stefan Hajnoczi <stefanha@redhat.com>
> 
> The fast path of cpu_physical_memory_sync_dirty_bitmap() directly
> manipulates the dirty bitmap.  Use atomic_xchg() to make the
> test-and-clear atomic.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> Message-Id: <1417519399-3166-7-git-send-email-stefanha@redhat.com>
> [Only do xchg on nonzero words. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Fam Zheng <famz@redhat.com>

> ---
>  include/exec/ram_addr.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index ea77187..0baba41 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -224,12 +224,12 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(unsigned long *dest,
>  
>          for (k = page; k < page + nr; k++) {
>              if (src[k]) {
> +                unsigned long bits = atomic_xchg(&src[k], 0);
>                  unsigned long new_dirty;
>                  new_dirty = ~dest[k];
> -                dest[k] |= src[k];
> -                new_dirty &= src[k];
> +                dest[k] |= bits;
> +                new_dirty &= bits;
>                  num_dirty += ctpopl(new_dirty);
> -                src[k] = 0;
>              }
>          }
>      } else {
> -- 
> 1.8.3.1
> 
>