Re: [Qemu-devel] [PATCH 1/4] block: Make bdrv_drain_invoke() recursive

[Kevin Wolf <kwolf@redhat.com>]

Am 05.12.2017 um 16:02 hat Paolo Bonzini geschrieben:
> On 05/12/2017 15:54, Kevin Wolf wrote:
> >      }
> >  
> > +    bdrv_drain_invoke(bs, true);
> >      bdrv_drain_recurse(bs, true);
> >  }
> >  
> > @@ -294,6 +298,7 @@ void bdrv_drained_end(BlockDriverState *bs)
> >      }
> >  
> >      bdrv_parent_drained_end(bs);
> > +    bdrv_drain_invoke(bs, false);
> >      bdrv_drain_recurse(bs, false);
> >      aio_enable_external(bdrv_get_aio_context(bs));
> 
> I think invoke should be done after recurse from bdrv_drain*end.  In the
> end aio_enable_external is a special kind of drain_end callback, so
> bdrv_drain_invoke should go together with it.

Yes and no.

Calling them together makes sense to me. Not for this patch, though,
which is just mechanical refactoring. bdrv_drain_invoke() was at the
beginning of bdrv_drain_recurse(), so it ends up before it after the
refactoring.

But there is really no reason at all to call bdrv_drain_recurse() in
bdrv_drained_end(). We only called it because it happened to involve
bdrv_drain_invoke(). After this patch, the call can be removed. I can do
that in a v2 (in a separate patch).

What we probably also should do is move bdrv_parent_drained_end() after
bdrv_drain_invoke() so that children are ready to accept new requests
when the parent callback is called. There is also some inconsistency
between bdrv_drain() and bdrv_drain_all() about the order of these
operations. I hope that in the end they would just call the same helper
and only the actual polling loop stays separate (as long as needed).

Kevin