linux-kernel - Re: [PATCH v2 3/4] KVM: Implement barriers before accessing kvm->buses[] on SRCU read paths

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Message-ID: <aHpgLnfQjp3qdZOL@google.com>
Date: Fri, 18 Jul 2025 07:54:38 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yao Yuan <yaoyuan@...ux.alibaba.com>
Cc: Keir Fraser <keirf@...gle.com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Eric Auger <eric.auger@...hat.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 3/4] KVM: Implement barriers before accessing
 kvm->buses[] on SRCU read paths

On Thu, Jul 17, 2025, Yao Yuan wrote:
> On Wed, Jul 16, 2025 at 11:07:36AM +0800, Keir Fraser wrote:
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 3bde4fb5c6aa..9132148fb467 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -965,11 +965,15 @@ static inline bool kvm_dirty_log_manual_protect_and_init_set(struct kvm *kvm)
> >  	return !!(kvm->manual_dirty_log_protect & KVM_DIRTY_LOG_INITIALLY_SET);
> >  }
> >
> > +/*
> > + * Get a bus reference under the update-side lock. No long-term SRCU reader
> > + * references are permitted, to avoid stale reads vs concurrent IO
> > + * registrations.
> > + */
> >  static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
> >  {
> > -	return srcu_dereference_check(kvm->buses[idx], &kvm->srcu,
> > -				      lockdep_is_held(&kvm->slots_lock) ||
> > -				      !refcount_read(&kvm->users_count));
> > +	return rcu_dereference_protected(kvm->buses[idx],
> > +					 lockdep_is_held(&kvm->slots_lock));
> 
> I want to consult the true reason for using protected version here,
> save unnecessary READ_ONCE() ?

Avoiding the READ_ONCE() is a happy bonus.  The main goal is to help document
and enforce that kvm_get_bus() can only be used if slots_lock is held.  Keeping
this as srcu_dereference_check() would result in PROVE_RCU getting a false negative
if the caller held kvm->srcu but not slots_lock.

>From a documentation perspective, rcu_dereference_protected() (hopefully) helps
highlight that there's something "special" about this helper, e.g. gives the reader
a hint that they probably shouldn't be using kvm_get_bus().