Re: [PATCH v8 02/13] vpci: use per-domain PCI lock to protect vpci structure

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

On Fri, Jul 21, 2023 at 08:02:33AM +0200, Jan Beulich wrote:
> On 20.07.2023 18:14, Roger Pau Monné wrote:
> >  Strictly speaking however the init
> > handlers don't require the lock in write mode unless we use such
> > locking to get exclusive access to all the devices assigned to the
> > domain BARs array for modify_bars().
> 
> Aiui in the present model modify_bars() has to use the vpci lock for
> protection.

But the current protection is insufficient, as we only hold the vpci
lock of the current device, but we don't hold the vpci lock of the
other devices when we iterate over in order to find overlapping bars
(or else it wold be an ABBA deadlock situation).

So my suggestion (which can be done later) is to take the newly
introduced per-domain rwlock in exclusive mode for modify_bars() in
order to assert there are no changes to the other devices vpci bar
fields.

> Therefore imo in any of the init functions the assertions
> should either express the real requirements of those functions, or be
> omitted on the basis that they're all called out of add-handlers
> anyway.

I'm happy to omit for the time being.  Iff we agree that modify_bars()
requires the rwlock in exclusive mode then we could add the assertion
there, but not in the init functions themselves.

Thanks, Roger.