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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan