linux-kernel - Re: [PATCH] KVM: use unified srcu interface function

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Message-ID: <Y5KNvgzakT1Vvxy4@google.com>
Date:   Fri, 9 Dec 2022 01:22:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Hao Peng <flyingpenghao@...il.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: use unified srcu interface function

On Thu, Dec 08, 2022, Hao Peng wrote:
> From: Peng Hao <flyingpeng@...cent.com>
> 
> kvm->irq_routing is protected by kvm->irq_srcu.
> 
> Signed-off-by: Peng Hao <flyingpeng@...cent.com>
> ---
>  virt/kvm/irqchip.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
> index 1e567d1f6d3d..90f54f04e37c 100644
> --- a/virt/kvm/irqchip.c
> +++ b/virt/kvm/irqchip.c
> @@ -216,7 +216,8 @@ int kvm_set_irq_routing(struct kvm *kvm,
>         }
> 
>         mutex_lock(&kvm->irq_lock);
> -       old = rcu_dereference_protected(kvm->irq_routing, 1);
> +       old = srcu_dereference_check(kvm->irq_routing, &kvm->irq_srcu,
> +                                       lockdep_is_held(&kvm->irq_lock));

Readers of irq_routing are protected via kvm->irq_srcu, but this writer is never
called with kvm->irq_srcu held.  I do like the of replacing '1' with
lockdep_is_held(&kvm->irq_lock) to document the protection, so what about just
doing that?  I.e.

diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
index 1e567d1f6d3d..77a18b4dc103 100644
--- a/virt/kvm/irqchip.c
+++ b/virt/kvm/irqchip.c
@@ -216,7 +216,8 @@ int kvm_set_irq_routing(struct kvm *kvm,
        }
 
        mutex_lock(&kvm->irq_lock);
-       old = rcu_dereference_protected(kvm->irq_routing, 1);
+       old = rcu_dereference_protected(kvm->irq_routing,
+                                       lockdep_is_held(&kvm->irq_lock));
        rcu_assign_pointer(kvm->irq_routing, new);
        kvm_irq_routing_update(kvm);
        kvm_arch_irq_routing_update(kvm);


>         rcu_assign_pointer(kvm->irq_routing, new);
>         kvm_irq_routing_update(kvm);
>         kvm_arch_irq_routing_update(kvm);
> --
> 2.27.0