Re: [Qemu-devel] [PATCH for-2.6 2/2] block/mirror: Refresh stale bitmap iterator cache

[Kevin Wolf]

Am 20.04.2016 um 00:59 hat Max Reitz geschrieben:
> If the drive's dirty bitmap is dirtied while the mirror operation is
> running, the cache of the iterator used by the mirror code may become
> stale and not contain all dirty bits.
> 
> This only becomes an issue if we are looking for contiguously dirty
> chunks on the drive. In that case, we can easily detect the discrepancy
> and just refresh the iterator if one occurs.
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block/mirror.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 2714a77..9df1fae 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -334,6 +334,11 @@ static uint64_t coroutine_fn 
> mirror_iteration(MirrorBlockJob *s)
>          }
>  
>          hbitmap_next = hbitmap_iter_next(&s->hbi);
> +        if (hbitmap_next > next_sector || hbitmap_next < 0) {
> +            /* The bitmap iterator's cache is stale, refresh it */
> +            bdrv_set_dirty_iter(&s->hbi, next_sector);
> +            hbitmap_next = hbitmap_iter_next(&s->hbi);
> +        }
>          assert(hbitmap_next == next_sector);

The iterator doesn't seem to be used afterwards anyway, so why not just
use next_sector and stop using the iterator when we already know what
result we want to get?

Kevin