Re: [PATCH v2 28/36] block: add bdrv_set_backing_noperm() transaction action

[Kevin Wolf]

Am 27.11.2020 um 15:45 hat Vladimir Sementsov-Ogievskiy geschrieben:
> Split out no-perm part of bdrv_set_backing_hd() as a separate
> transaction action. Note the in case of existing BdrvChild we reuse it,
> not recreate, just to do less actions.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

>  /*
>   * Sets the bs->backing link of a BDS. A new reference is created; callers
>   * which don't need their own reference any more must call bdrv_unref().
>   */
> -void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
> -                         Error **errp)
> +static int bdrv_set_backing_noperm(BlockDriverState *bs,
> +                                   BlockDriverState *backing_bs,
> +                                   GSList **tran, Error **errp)
>  {
> -    bool update_inherits_from = bdrv_chain_contains(bs, backing_hd) &&
> -        bdrv_inherits_from_recursive(backing_hd, bs);
> +    int ret = 0;
> +    bool update_inherits_from = bdrv_chain_contains(bs, backing_bs) &&
> +        bdrv_inherits_from_recursive(backing_bs, bs);
> +    GSList *attach_tran = NULL;
> +    BdrvSetBackingNoPermState *s;
>  
>      if (bdrv_is_backing_chain_frozen(bs, child_bs(bs->backing), errp)) {
> -        return;
> +        return -EPERM;
>      }
>  
> -    if (backing_hd) {
> -        bdrv_ref(backing_hd);
> +    if (bs->backing && backing_bs) {
> +        bdrv_replace_child_safe(bs->backing, backing_bs, tran);

The old code with separate bdrv_unref_child() and then
bdrv_attach_child() tried to make the AioContests of bs and backing_bs
compatible by moving one of the nodes if necessary.

bdrv_replace_child_safe() doesn't seem to do that, but it only asserts
that both nodes are already in the same context.

I see that iotest 245 doesn't crash, which I think it should if this
were broken, but where does the switch happen now?

Kevin