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

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

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.

I have some comments about the model in patch 2, I think we need to
clarify the intended usage on the commit log about pdev and refcounts.

Thanks, Roger.