linux-kernel - Re: [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain

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Message-ID: <Y1K1AqVWEyY0/Uqy@nvidia.com>
Date:   Fri, 21 Oct 2022 12:04:34 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Gerd Bayer <gbayer@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain_list
 iteration

On Fri, Oct 21, 2022 at 05:01:32PM +0200, Niklas Schnelle wrote:
> On Fri, 2022-10-21 at 10:36 -0300, Jason Gunthorpe wrote:
> > 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
> 
> I'd tend to go with synchronize_rcu(). It won't leave us with spurious
> error logs for the failed IOTLB flushes and as you said one expects
> detach to be synchronous. I don't think waiting in it will be a
> problem. But this is definitely something you're more of an expert on
> so I'll trust your judgement. Looking at other callers of
> synchronize_rcu() quite a few of them look to be in similar
> detach/release kind of situations though not sure how frequent and
> performance critical IOMMU domain detaching is in comparison.

I would not do it on domain detaching, that is something triggered by
userspace through VFIO and it could theoritically happen alot, eg in
vIOMMU scenarios.

Jason