Re: [PATCH v3 1/6] xen: add reference counter support

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 16, 2023 at 05:56:00PM +0100, Jan Beulich wrote:
> On 16.03.2023 17:48, Roger Pau Monné wrote:
> > On Thu, Mar 16, 2023 at 05:43:18PM +0100, Jan Beulich wrote:
> >> On 16.03.2023 17:39, Roger Pau Monné wrote:
> >>> On Thu, Mar 16, 2023 at 05:32:38PM +0100, Jan Beulich wrote:
> >>>> On 16.03.2023 17:19, Roger Pau Monné wrote:
> >>>>> On Tue, Mar 14, 2023 at 08:56:29PM +0000, Volodymyr Babchuk wrote:
> >>>>>> +static inline void refcnt_get(refcnt_t *refcnt)
> >>>>>> +{
> >>>>>> +    int old = atomic_add_unless(&refcnt->refcnt, 1, 0);
> >>>>>
> >>>>> Occurred to me while looking at the next patch:
> >>>>>
> >>>>> Don't you also need to print a warning (and saturate the counter
> >>>>> maybe?) if old == 0, as that would imply the caller is attempting
> >>>>> to take a reference of an object that should be destroyed? IOW: it
> >>>>> would point to some kind of memory leak.
> >>>>
> >>>> Hmm, I notice the function presently returns void. I think what to do
> >>>> when the counter is zero needs leaving to the caller. See e.g.
> >>>> get_page() which will simply indicate failure to the caller in case
> >>>> the refcnt is zero. (There overflow handling also is left to the
> >>>> caller ... All that matters is whether a ref can be acquired.)
> >>>
> >>> Hm, likely.  I guess pages never go away even when it's refcount
> >>> reaches 0.
> >>>
> >>> For the pdev case attempting to take a refcount on an object that has
> >>> 0 refcounts implies that the caller is using leaked memory, as the
> >>> point an object reaches 0 it supposed to be destroyed.
> >>
> >> Hmm, my thinking was that a device would remain at refcnt 0 until it is
> >> actually removed, i.e. refcnt == 0 being a prereq for pci_remove_device()
> >> to be willing to do anything at all. But maybe that's not a viable model.
> > 
> > Right, I think the intention was for pci_remove_device() to drop the
> > refcount to 0 and do the removal, so the refcount should be 1 when
> > calling pci_remove_device().  But none of this is written down, so
> > it's mostly my assumptions from looking at the code.
> 
> Could such work at all? The function can't safely drop a reference
> and _then_ check whether it was the last one. The function either has
> to take refcnt == 0 as prereq, or it needs to be the destructor
> function that refcnt_put() calls.

But then you also get in the trouble of asserting that refcnt == 0
doesn't change between evaluation and actual removal of the structure.

Should all refcounts to pdev be taken and dropped while holding the
pcidevs lock?

I there an email (outside of this series) that contains a description
of how the refcounting is to be used with pdevs?

Thanks, Roger.