Re: [Qemu-devel] [PATCH 3/3] block: remove legacy I/O throttling

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Aug 01, 2017 at 04:49:07PM +0300, Manos Pitsidianakis wrote:
> diff --git a/block.c b/block.c
> index 9ebdba28b0..c6aad25286 100644
> --- a/block.c
> +++ b/block.c
> @@ -1975,6 +1975,7 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
>      child = g_new(BdrvChild, 1);
>      *child = (BdrvChild) {
>          .bs             = NULL,
> +        .parent_bs      = NULL,
>          .name           = g_strdup(child_name),
>          .role           = child_role,
>          .perm           = perm,
> @@ -2009,6 +2010,7 @@ BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
>      if (child == NULL) {
>          return NULL;
>      }
> +    child->parent_bs = parent_bs;
>  
>      QLIST_INSERT_HEAD(&parent_bs->children, child, next);
>      return child;
> @@ -3729,6 +3731,12 @@ const char *bdrv_get_parent_name(const BlockDriverState *bs)
>                  return name;
>              }
>          }
> +        if (c->parent_bs && c->parent_bs->implicit) {
> +            name = bdrv_get_parent_name(c->parent_bs);
> +            if (name && *name) {
> +                return name;
> +            }
> +        }
>      }
>  
>      return NULL;

This should be a separate patch.

Who updates parent_bs if the parent is changed (e.g.
bdrv_replace_node())?

We already have bs->parents.  Why is BdrvChild->parent_bs needed?

> -void blk_io_limits_disable(BlockBackend *blk)
> +void blk_io_limits_disable(BlockBackend *blk, Error **errp)
>  {
> -    assert(blk->public.throttle_group_member.throttle_state);
> -    bdrv_drained_begin(blk_bs(blk));

Is it safe to drop drained_begin?  We must ensure that no I/O requests
run during this function.

> -    throttle_group_unregister_tgm(&blk->public.throttle_group_member);
> -    bdrv_drained_end(blk_bs(blk));
> +    BlockDriverState *bs, *throttle_node;
> +
> +    throttle_node = blk_get_public(blk)->throttle_node;

Is blk_get_public() still necessary?  Perhaps we can do away with the
concept of the public struct now.  It doesn't need to be done in this
patch though.

> +
> +    assert(throttle_node && throttle_node->refcnt == 1);

Are you sure the throttle_node->refcnt == 1 assertion holds?  For
example, does the built-in NBD server have a reference to the throttle
node if nbd-server-add is called after throttling has been enabled?

Since we have the blk->throttle_node pointer we know we're the owner.
Others may be using the node too but we may choose to remove it at any
time.

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