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

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

On Mon, Feb 07, 2022 at 03:11:03PM +0100, Jan Beulich wrote:
> On 07.02.2022 14:53, Oleksandr Andrushchenko wrote:
> > On 07.02.22 14:46, Roger Pau Monné wrote:
> >> I think the per-domain rwlock seems like a good option. I would do
> >> that as a pre-patch.
> > It is. But it seems it won't solve the thing we started this adventure for:
> > 
> > With per-domain read lock and still ABBA in modify_bars (hope the below
> > is correctly seen with a monospace font):
> > 
> > cpu0: vpci_write-> d->RLock -> pdev1->lock ->                               
> >                    rom_write -> modify_bars: tmp (pdev2) ->lock
> > cpu1:        vpci_write-> d->RLock pdev2->lock -> cmd_write -> modify_bars: 
> > tmp (pdev1) ->lock
> > 
> > There is no API to upgrade read lock to write lock in modify_bars which 
> > could help,
> > so in both cases vpci_write should take write lock.
> 
> Hmm, yes, I think you're right: It's not modify_bars() itself which needs
> to acquire the write lock, but its (perhaps indirect) caller. Effectively
> vpci_write() would need to take the write lock if the range written
> overlaps the BARs or the command register.

I'm confused. If we use a per-domain rwlock approach there would be no
need to lock tmp again in modify_bars, because we should hold the
rwlock in write mode, so there's no ABBA?

We will have however to drop the per domain read and vpci locks and
pick the per-domain lock in write mode.

Thanks, Roger.