linux-kernel - Re: [PATCH 2/2] KVM: Fix rcu splat if vm creation fails

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Date:   Mon, 4 Nov 2019 20:25:11 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 2/2] KVM: Fix rcu splat if vm creation fails

On Mon, 4 Nov 2019 at 20:21, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 04/11/19 13:16, Wanpeng Li wrote:
> >> I don't understand this one, hasn't
> >>
> >>         WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
> >>
> >> decreased the conut already?  With your patch the refcount would then
> >> underflow.
> >
> > r = kvm_arch_init_vm(kvm, type);
> > if (r)
> >     goto out_err_no_arch_destroy_vm;
> >
> > out_err_no_disable:
> >     kvm_arch_destroy_vm(kvm);
> >     WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
> > out_err_no_arch_destroy_vm:
> >
> > So, if kvm_arch_init_vm() fails, we will not execute
> > WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
>
> Uuh of course.  But I'd rather do the opposite: move the refcount_set
> earlier so that refcount_dec_and_test can be moved after
> no_arch_destroy_vm.  Moving the refcount_set is not strictly necessary,
> but avoids the introduction of yet another label.
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e22ff63e5b1a..e7a07132cd7f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -650,6 +650,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
>         if (init_srcu_struct(&kvm->irq_srcu))
>                 goto out_err_no_irq_srcu;
>
> +       refcount_set(&kvm->users_count, 1);
>         for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
>                 struct kvm_memslots *slots = kvm_alloc_memslots();
>
> @@ -667,7 +668,6 @@ static struct kvm *kvm_create_vm(unsigned long type)
>                         goto out_err_no_arch_destroy_vm;
>         }
>
> -       refcount_set(&kvm->users_count, 1);
>         r = kvm_arch_init_vm(kvm, type);
>         if (r)
>                 goto out_err_no_arch_destroy_vm;
> @@ -696,8 +696,8 @@ static struct kvm *kvm_create_vm(unsigned long type)
>         hardware_disable_all();
>  out_err_no_disable:
>         kvm_arch_destroy_vm(kvm);
> -       WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
>  out_err_no_arch_destroy_vm:
> +       WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
>         for (i = 0; i < KVM_NR_BUSES; i++)
>                 kfree(kvm_get_bus(kvm, i));
>         for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)

Good point, I will handle these two patches later.

    Wanpeng