Re: [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain_list iteration

Jason Gunthorpe
2022-10-21T13:36:37Z

On Fri, Oct 21, 2022 at 02:08:02PM +0200, Niklas Schnelle wrote:
> On Thu, 2022-10-20 at 08:05 -0300, Jason Gunthorpe wrote:
> > On Thu, Oct 20, 2022 at 10:51:10AM +0200, Niklas Schnelle wrote:
> >
> > > Ok that makes sense thanks for the explanation. So yes my assessment is
> > > still that in this situation the IOTLB flush is architected to return
> > > an error that we can ignore. Not the most elegant I admit but at least
> > > it's simple. Alternatively I guess we could use call_rcu() to do the
> > > zpci_unregister_ioat() but I'm not sure how to then make sure that a
> > > subsequent zpci_register_ioat() only happens after that without adding
> > > too much more logic.
> >
> > This won't work either as the domain could have been freed before the
> > call_rcu() happens, the domain needs to be detached synchronously
> >
> > Jason
>
> Yeah right, that is basically the same issue I was thinking of for a
> subsequent zpci_register_ioat(). What about the obvious one. Just call
> synchronize_rcu() before zpci_unregister_ioat()?

Ah, it can be done, but be prepared to wait >> 1s for synchronize_rcu
to complete in some cases.

What you have seems like it could be OK, just deal with the ugly racy
failure

Jason