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.
