linux-kernel - Re: [PATCH v13 21/21] KVM: pfncache: rework __kvm_gpc

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Message-ID: <df6ad8b9-4e53-4357-ab17-e9af62342849@xen.org>
Date: Fri, 16 Feb 2024 14:03:33 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.org>, Paolo Bonzini
 <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>,
 David Hildenbrand <david@...hat.com>, Heiko Carstens <hca@...ux.ibm.com>,
 Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>,
 Sean Christopherson <seanjc@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-s390@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v13 21/21] KVM: pfncache: rework __kvm_gpc_refresh() to
 fix locking issues

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@...zon.co.uk>
>>
>> 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.
> 
> 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.

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?