Re: [PATCH v13 21/21] KVM: pfncache: rework __kvm_gpc_refresh() to fix locking issues

[Sean Christopherson]

On Fri, Feb 16, 2024, Paul Durrant wrote:
> On 16/02/2024 13:04, David Woodhouse wrote:
> > On Thu, 2024-02-15 at 15:29 +0000, Paul Durrant wrote:
> > > From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > > 
> > > This function can race with kvm_gpc_deactivate(), which does not take
> > > the ->refresh_lock. This means kvm_gpc_deactivate() can wipe the ->pfn
> > > and ->khva fields, and unmap the latter, while hva_to_pfn_retry() has
> > > temporarily dropped its write lock on gpc->lock.
> > 
> > Let's drop this from your series for now, as it's contentious.
> > 
> > Sean didn't like calling it a 'fix', which I had conceded and reworked
> > the commit message. It was on the list somewhere, and also in
> > https://git.infradead.org/users/dwmw2/linux.git/commitdiff/f19755000a7
> > 
> > I *also* think we should do this simpler one:
> > https://git.infradead.org/users/dwmw2/linux.git/commitdiff/cc69506d19a
> > ... which almost makes the first one unnecessary, but I think we should
> > do it *anyway* because the rwlock abuse it fixes is kind of awful.
> > 
> > And while we still can't actually *identify* the race condition that
> > led to a dereference of a NULL gpc->khva while holding the read lock
> > and gpc->valid and gpc->active both being true... I'll eat my hat if
> > cleaning up and simplifying the locking (and making it self-contained)
> > *doesn't* fix it.

Heh, I'm not taking that bet.

> > But either way, it isn't really part of your series. The only reason it
> > was tacked on the end was because it would have merge conflicts with
> > your series, which had been outstanding for months already.
> > 
> > So drop this one, and I'll work this bit out with Sean afterwards.

FWIW, I'm not opposed to overhauling the gpc locking, I agree it's a mess.  I just
want to proceed slower than I would for a fix, it's a lot to digest.

> Ok. Sean, I assume that since this is the last patch in the series it's
> superfluous for me to post a v14 just for this?

Correct, definitely no need for a new version.