Re: [Qemu-devel] [PATCH qemu] pci: Add missing drop of bus master AS reference

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 28, 2017 at 11:06:28AM +1100, David Gibson wrote:
> On Mon, Mar 27, 2017 at 05:28:17PM +0200, Paolo Bonzini wrote:
> > 
> > 
> > On 27/03/2017 06:40, Alexey Kardashevskiy wrote:
> > > The recent introduction of a bus master container added
> > > memory_region_add_subregion() into the PCI device registering path but
> > > missed memory_region_del_subregion() in the unregistering path leaving
> > > a reference to the root memory region of the new container.
> > > 
> > > This adds missing memory_region_del_subregion().
> > > 
> > > Fixes: 3716d5902d743 ("pci: introduce a bus master container")
> > > Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> > > ---
> > >  hw/pci/pci.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> > > index e6b08e1988..bd8043c460 100644
> > > --- a/hw/pci/pci.c
> > > +++ b/hw/pci/pci.c
> > > @@ -869,6 +869,8 @@ static void do_pci_unregister_device(PCIDevice *pci_dev)
> > >      pci_dev->bus->devices[pci_dev->devfn] = NULL;
> > >      pci_config_free(pci_dev);
> > >  
> > > +    memory_region_del_subregion(&pci_dev->bus_master_container_region,
> > > +                                &pci_dev->bus_master_enable_region);
> > >      address_space_destroy(&pci_dev->bus_master_as);
> > >  }
> > >  
> > > 
> > 
> > My own review fail.  The enable subregion would be deleted when a memory
> > region is finalized, but the enable subregions is keeping the owner
> > alive.  And until the owner is alive, the container region is not
> > deleted either.  So there is a reference count cycle, which we need to
> > break.
> > 
> > It's probably good to revisit commit 2e2b8eb ("memory: allow destroying
> > a non-empty MemoryRegion", 2015-10-01).  For 2.9,
> > 
> > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> Merged to ppc-for-2.9.

I have it on pci branch, seems more appropriate.

> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson