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

[Kevin Wolf <kwolf@redhat.com>]

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Am 02.08.2017 um 12:07 hat Stefan Hajnoczi geschrieben:
> 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?

I haven't look at the whole patch yet, but BdrvChild->parent_bs is a
thing that intentionally doesn't exist. A node simply has no business
knowing its parents - which may or may not be BlockDriverStates (the
obvious example where they aren't BDSes are BlockBackends, but block
jobs own some BdrvChild objects, too).

Usually the replacement is a BdrvChildRole callback.

Kevin

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