Re: [PATCH] vpci: introduce per-domain lock to protect vpci structure

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 11.02.22 17:44, Roger Pau Monné wrote:
> On Fri, Feb 11, 2022 at 12:13:38PM +0000, Oleksandr Andrushchenko wrote:
>>
>> On 11.02.22 13:40, Roger Pau Monné wrote:
>>> On Fri, Feb 11, 2022 at 07:27:39AM +0000, Oleksandr Andrushchenko wrote:
>>>> Hi, Roger!
>>>>
>>>> On 10.02.22 18:16, Roger Pau Monné wrote:
>>>>> On Wed, Feb 09, 2022 at 03:36:27PM +0200, Oleksandr Andrushchenko wrote:
>>>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>>>>>>
>>>>>> Introduce a per-domain read/write lock to check whether vpci is present,
>>>>>> so we are sure there are no accesses to the contents of the vpci struct
>>>>>> if not. This lock can be used (and in a few cases is used right away)
>>>>>> so that vpci removal can be performed while holding the lock in write
>>>>>> mode. Previously such removal could race with vpci_read for example.
>>>>> Sadly there's still a race in the usage of pci_get_pdev_by_domain wrt
>>>>> pci_remove_device, and likely when vPCI gets also used in
>>>>> {de}assign_device I think.
>>>> Yes, this is indeed an issue, but I was not trying to solve it in
>>>> context of vPCI locking yet. I think we should discuss how do
>>>> we approach pdev locking, so I can create a patch for that.
>>>> that being said, I would like not to solve pdev in  this patch yet
>>>>
>>>> ...I do understand we do want to avoid that, but at the moment
>>>> a single reliable way for making sure pdev is alive seems to
>>>> be pcidevs_lock....
>>> I think we will need to make pcidevs_lock a rwlock and take it in read
>>> mode for pci_get_pdev_by_domain.
>>>
>>> We didn't have this scenario before where PCI emulation is done in the
>>> hypervisor, and hence the locking around those data structures has not
>>> been designed for those use-cases.
>> Yes, I do understand that.
>> I hope pcidevs lock move to rwlock can be done as a separate
>> patch. While this is not done, do you think we can proceed with
>> vPCI series and pcidevs locking re-work being done after?
> Ideally we would like to sort out the locking once and for all. I
> would like to be sure that what we introduce now doesn't turn out to
> interact badly when we decide to look at the pcidevs locking issue.
Ok, so I'll start converting pcidevs into rwlock then
>
> Thanks, Roger.
Thank you,
Oleksandr