Re: [Qemu-devel] [PATCH v3 0/7] Implement reference count for BlockDriverState

[Fam Zheng <famz@redhat.com>]

On Thu, 08/01 14:28, Stefan Hajnoczi wrote:
> On Wed, Jul 31, 2013 at 06:13:53PM +0800, Fam Zheng wrote:
> > BlockDriverState lifecycle management is needed by future features such as
> > image fleecing and blockdev-add. This series adds reference count to
> > BlockDriverState.
> > 
> > The first two patches clean up two odd BlockDriverState use cases, so all code
> > uses bdrv_new() to create BlockDriverState instance.
> > 
> > Then implemented bdrv_ref() and bdrv_unref() to operate on refcnt: Initially,
> > refcnt is 1, which means bdrv_unref is effectively a bdrv_delete() here. So
> > patch 04 has a search and replace to convert bdrv_delete to bdrv_unref, before
> > bdrv_ref is used anywhere. 05~08 patches calls bdrv_ref for device attach,
> > block-migration and nbd.
> > 
> > The rule is: Either bdrv_ref() or bdrv_new() must have a matching
> > bdrv_unref() call, and the last matching bdrv_unref deletes the bs.
> > 
> > v3:
> >     03: Removed unnecessary bdrv_close() call.
> > 
> > v2:
> >     05: Removed: "block: use BlockDriverState refcnt for device attach/detach"
> >     07: Fix xen_disk blk_disconnect() as it depended on device attach refcnt.
> > 
> > Fam Zheng (7):
> >   vvfat: use bdrv_new() to allocate BlockDriverState
> >   iscsi: use bdrv_new() instead of stack structure
> >   block: implement reference count for BlockDriverState
> >   block: make bdrv_delete() static
> >   migration: omit drive ref as we have bdrv_ref now
> >   xen_disk: simplify blk_disconnect with refcnt
> >   nbd: use BlockDriverState refcnt
> 
> Follow-up question:
> 
> Did you look at using bdrv_ref() for the BDS <- BlockJob relationship
> too?
> 
> blockdev.c block job code still uses the DriveInfo refcount after your
> series.  The BDS reference would be sufficient since the DriveInfo
> fields are not used by block jobs.
> 
For now they coexist, which works but with redundancy. I will send
follow-up patch(es) to clean up block job to only use BDS refcount.

-- 
Fam