Re: [PATCH v6 03/13] vpci: move lock outside of struct vpci

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 07.02.22 14:34, Jan Beulich wrote:
> On 07.02.2022 12:08, Oleksandr Andrushchenko wrote:
>> 1. vpci_{read|write} are not protected with pcidevs_lock and can run in
>> parallel with pci_remove_device which can remove pdev after vpci_{read|write}
>> acquired the pdev pointer. This may lead to a fail due to pdev dereference.
>>
>> So, to protect pdev dereference vpci_{read|write} must also use pdevs_lock.
> I think this is not the only place where there is a theoretical race
> against pci_remove_device().
Not at all, that was just to demonstrate one of the possible sources of races.
>   I would recommend to separate the
> overall situation with pcidevs_lock from the issue here.
Do you agree that there is already an issue with that? In the currently 
existing code?
>   I don't view
> it as an option to acquire pcidevs_lock in vpci_{read,write}().
Yes, that would hurt too much, I agree. But this needs to be solved
>   If
> anything, we need proper refcounting of PCI devices (at which point
> likely a number of lock uses can go away).
It seems so. Then not only pdev's need refcounting, but pdev->vpci as well

What's your view on how can we achieve both goals?
pdev and pdev->vpci and locking/refcounting
This is really crucial for all the code for PCI passthrough on Arm because
without this ground work done we can't accept all the patches which rely
on this: vPCI changes, MSI/MSI-X etc.
>
> Jan
>
Thank you,
Oleksandr