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

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Message-ID: <9e88b51b-9b37-3f14-8d98-0bb3ddbe6378@redhat.com>
Date:   Mon, 4 Nov 2019 13:26:57 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.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 04/11/19 13:25, Wanpeng Li wrote:
> 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.

No problem, I can send v2 after testing.

Paolo