Re: [PATCH 1/7] stream: Traverse graph after modification

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

04.11.2021 13:38, Hanna Reitz wrote:
> bdrv_cor_filter_drop() modifies the block graph.  That means that other
> parties can also modify the block graph before it returns.  Therefore,
> we cannot assume that the result of a graph traversal we did before
> remains valid afterwards.
> 
> We should thus fetch `base` and `unfiltered_base` afterwards instead of
> before.
> 
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>

The fact that other parties can modify graph during our graph modification is a global problem.. The patch doesn't fix it, but reduces its effect in specific case.. OK.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>


> ---
>   block/stream.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/block/stream.c b/block/stream.c
> index 97bee482dc..e45113aed6 100644
> --- a/block/stream.c
> +++ b/block/stream.c
> @@ -54,8 +54,8 @@ static int stream_prepare(Job *job)
>   {
>       StreamBlockJob *s = container_of(job, StreamBlockJob, common.job);
>       BlockDriverState *unfiltered_bs = bdrv_skip_filters(s->target_bs);
> -    BlockDriverState *base = bdrv_filter_or_cow_bs(s->above_base);
> -    BlockDriverState *unfiltered_base = bdrv_skip_filters(base);
> +    BlockDriverState *base;
> +    BlockDriverState *unfiltered_base;
>       Error *local_err = NULL;
>       int ret = 0;
>   
> @@ -63,6 +63,9 @@ static int stream_prepare(Job *job)
>       bdrv_cor_filter_drop(s->cor_filter_bs);
>       s->cor_filter_bs = NULL;
>   
> +    base = bdrv_filter_or_cow_bs(s->above_base);
> +    unfiltered_base = bdrv_skip_filters(base);
> +
>       if (bdrv_cow_child(unfiltered_bs)) {
>           const char *base_id = NULL, *base_fmt = NULL;
>           if (unfiltered_base) {
> 


-- 
Best regards,
Vladimir