Re: [PATCH v7 03/12] PCI: liveupdate: Track incoming preserved PCI devices

[David Matlack]

On Tue, Jul 21, 2026 at 4:02 PM Samiullah Khawaja <skhawaja@xxxxxxxxxx> wrote:
>
> On Mon, Jul 20, 2026 at 02:54:51PM -0700, David Matlack wrote:
> >On Fri, Jul 17, 2026 at 2:47 PM Pasha Tatashin
> ><pasha.tatashin@xxxxxxxxxx> wrote:
> >>
> >> On Fri, 10 Jul 2026 21:26:06 +0000, David Matlack <dmatlack@xxxxxxxxxx> wrote:
> >> > diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
> >> > index 03075ce06ac9..df6a02240aa4 100644
> >> > --- a/drivers/pci/liveupdate.c
> >> > +++ b/drivers/pci/liveupdate.c
> >> > @@ -298,6 +377,87 @@ void pci_liveupdate_unpreserve(struct pci_dev *dev)
> >> > [ ... skip 76 lines ... ]
> >> > +     /*
> >> > +      * Hold the ref on the incoming FLB until pci_liveupdate_finish() so
> >> > +      * that dev->liveupdate.incoming cannot get freed while the PCI core
> >> > +      * has a pointer to it. It's better to leak the incoming FLB than do a
> >> > +      * use-after-free if driver does not call pci_liveupdate_finish().
> >> > +      */
> >>
> >> I am confused by this. Each preserved PCI device must have an associated
> >> FD preserved with it via LUO. I.e., vfiofd would need to be preserved. If
> >> the vfiofd was not reclaimed, and finish is not possible, that vfiofd
> >> would still be owned by LUO, and therefore PCI FLB refcount would stay
> >> positive.
> >>
> >> However, if finish is possible, and this is the last vfiofd that is
> >> finished, FLB will be freed as soon as the reference count reaches zero,
> >> which I would think is the expected behavior.
> >>
> >> What is the point of holding a reference here, instead of only for the
> >> duration of FLB access, i.e. to make sure we are accessing a valid data?
> >
> >The duration of the access is from here until pci_liveupdate_finish()
> >because that it when the pointer (dev->liveupdate.incoming) is
> >cleared. So that is why the PCI core holds the reference from here
> >until pci_liveupdate_finish().
>
> I think LUO gives you guarantee that this pointer remains valid until
> the FLB finish(), because all the vfiofd that were preserved have not
> finished. And when all vfiofd have finished, LUO lets you know that the
> pointer is becoming invalid so you can unset these in the finish()
> callback from LUO?
>
> If I understand this correctly, I think by keeping long references until
> finish you are replicating the vfiofd bound lifecycle that LUO already
> gives you.

Yes that's right. I am taking the approach of not trusting drivers
(i.e. VFIO) and having the PCI core maintain it's own references so
that FLB finish() is guaranteed to never be called while a device has
device->liveupdate.incoming set.

Alternatively, the PCI core could avoid taking a reference and instead
check for driver bugs in FLB finish(). I previously explored that
approach and ran into a circular lock dependency between the FLB mutex
and pci_liveupdate.rwsem. But I can look into that again, or just do a
lockless walk of all PCI devices and panic if any still has
dev->liveupdate.incoming set. It will only happen if the driver is
completely broken so panicking seems fine.