Re: [Qemu-devel] [PATCH v14 09/14] stream: Use bdrv_drop_intermediate and drop close_unused_images

[Jeff Cody <jcody@redhat.com>]

On Wed, Feb 19, 2014 at 09:42:26PM +0800, Fam Zheng wrote:
> This reuses the new bdrv_drop_intermediate.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/stream.c | 30 +-----------------------------
>  1 file changed, 1 insertion(+), 29 deletions(-)
> 
> diff --git a/block/stream.c b/block/stream.c
> index dd0b4ac..9cdcf0e 100644
> --- a/block/stream.c
> +++ b/block/stream.c
> @@ -51,34 +51,6 @@ static int coroutine_fn stream_populate(BlockDriverState *bs,
>      return bdrv_co_copy_on_readv(bs, sector_num, nb_sectors, &qiov);
>  }
>  
> -static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
> -                                const char *base_id)
> -{
> -    BlockDriverState *intermediate;
> -    intermediate = top->backing_hd;
> -
> -    /* Must assign before bdrv_delete() to prevent traversing dangling pointer
> -     * while we delete backing image instances.
> -     */
> -    top->backing_hd = base;
> -
> -    while (intermediate) {
> -        BlockDriverState *unused;
> -
> -        /* reached base */
> -        if (intermediate == base) {
> -            break;
> -        }
> -
> -        unused = intermediate;
> -        intermediate = intermediate->backing_hd;
> -        unused->backing_hd = NULL;
> -        bdrv_unref(unused);
> -    }
> -
> -    bdrv_refresh_limits(top);
> -}
> -
>  static void coroutine_fn stream_run(void *opaque)
>  {
>      StreamBlockJob *s = opaque;
> @@ -192,7 +164,7 @@ wait:
>              }
>          }


>          ret = bdrv_change_backing_file(bs, base_id, base_fmt);

This is redundant if bdrv_drop_intermediate() is used, and can be
removed.

> -        close_unused_images(bs, base, base_id);
> +        bdrv_drop_intermediate(bs, bs->backing_hd, base);

Previously, if base == NULL and base_id == NULL, that meant that we
were streaming from the very first backing file all the way up to the
active layer.  When close_unused_images() was called with base ==
NULL, the BDS chain was flattened and the active layer became all that
was left.

By calling bdrv_drop_intermediate() with base == NULL, we get the same
behavior, so that is OK.

What about if base != NULL here?  It still works OK, but now 'base' is
really 'bs->backing_hd'.  But I don't think we care anymore in this
function, or in the streaming block job, so I think it is OK.



>      }
>  
>      qemu_vfree(buf);
> -- 
> 1.8.5.4
> 
>