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

[Samiullah Khawaja <skhawaja@google.com>]

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@soleen.com> wrote:
>>
>> On Fri, 10 Jul 2026 21:26:06 +0000, David Matlack <dmatlack@google.com> 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.
>
>We could avoid this by deleteing dev->liveupdate.incoming and,
>instead, fetching the incoming FLB and doing the xarray lookup every
>atime the PCI core needs to access the device's incoming ser struct,
>but that would be inefficient.

Sami