Re: [Qemu-devel] [PATCH v4 04/11] block: improve should_update_child

[Kevin Wolf <kwolf@redhat.com>]

Am 15.10.2018 um 18:06 hat Vladimir Sementsov-Ogievskiy geschrieben:
> As it already said in the comment, we don't want to create loops in
> parent->child relations. So, when we try to append @to to @c, we should
> check that @c is not in @to children subtree, and we should check it
> recursively, not only the first level. The patch provides BFS-based
> search, to check the relations.
> 
> This is needed for further fleecing-hook filter usage: we need to
> append it to source, when the hook is already a parent of target, and
> source may be in a backing chain of target (fleecing-scheme). So, on
> appending, the hook should not became a child (direct or through
> children subtree) of the target.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block.c | 32 +++++++++++++++++++++++++++-----
>  1 file changed, 27 insertions(+), 5 deletions(-)
> 
> diff --git a/block.c b/block.c
> index c298ca6a19..7f605b0bf0 100644
> --- a/block.c
> +++ b/block.c
> @@ -3432,7 +3432,7 @@ void bdrv_close_all(void)
>  
>  static bool should_update_child(BdrvChild *c, BlockDriverState *to)
>  {
> -    BdrvChild *to_c;
> +    GList *queue = NULL, *pos;
>  
>      if (c->role->stay_at_node) {
>          return false;
> @@ -3468,13 +3468,35 @@ static bool should_update_child(BdrvChild *c, BlockDriverState *to)
>       * if A is a child of B, that means we cannot replace A by B there
>       * because that would create a loop.  Silently detaching A from B
>       * is also not really an option.  So overall just leaving A in
> -     * place there is the most sensible choice. */
> -    QLIST_FOREACH(to_c, &to->children, next) {
> -        if (to_c == c) {
> -            return false;
> +     * place there is the most sensible choice.
> +     *
> +     * upd: If the child @c belongs to the @to's children, or children of it's
> +     * children and so on - this would create a loop to. To prevent it let's do
> +     * a BFS search on @to children subtree.
> +     */

I don't like the "upd:" thing. Let me try to rephrase the addition:

    We would also create a loop in any cases where @c is only indirectly
    referenced by @to. Prevent this by returning false if @c is found
    (by breadth-first search) anywhere in the whole subtree of @to.

Also, even though the coding style says 80 characters per line, the
existing comment seems to use only 70 characters. Let's try to stay
consistent here, otherwise it looks a bit odd.

> +    pos = queue = g_list_append(queue, to);
> +    while (pos) {
> +        BlockDriverState *v = pos->data;
> +        BdrvChild *c2;
> +
> +        QLIST_FOREACH(c2, &v->children, next) {
> +            if (c2 == c) {
> +                g_list_free(queue);
> +                return false;
> +            }
> +
> +            if (g_list_find(queue, c2->bs)) {
> +                continue;
> +            }
> +
> +            queue = g_list_append(queue, c2->bs);
>          }
> +
> +        pos = pos->next;
>      }
>  
> +    g_list_free(queue);
>      return true;
>  }

The functional change looks good to me.

Kevin