Re: [PATCH] KVM/Eventfd: Avoid crash when assign and deassign same eventfd in parallel.
From: Paolo Bonzini
Date: Mon Dec 18 2017 - 04:25:08 EST
On 18/12/2017 10:08, David Hildenbrand wrote:
> On 18.12.2017 09:50, Paolo Bonzini wrote:
>> On 18/12/2017 09:30, David Hildenbrand wrote:
>>> The ugly thing in kvm_irqfd_assign() is that we access irqfd without
>>> holding a lock. I think that should rather be fixed than working around
>>> that issue. (e.g. lock() -> lookup again -> verify still in list ->
>> I wonder if it's even simpler:
>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> index f2ac53ab8243..17ed298bd66f 100644
>> --- a/virt/kvm/eventfd.c
>> +++ b/virt/kvm/eventfd.c
>> @@ -387,7 +387,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
>> idx = srcu_read_lock(&kvm->irq_srcu);
>> irqfd_update(kvm, irqfd);
>> - srcu_read_unlock(&kvm->irq_srcu, idx);
>> list_add_tail(&irqfd->list, &kvm->irqfds.items);
>> @@ -420,10 +419,12 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
>> irqfd->consumer.token, ret);
>> + srcu_read_unlock(&kvm->irq_srcu, idx);
> Was worried about the poll() call. But if that works, it would be very nice.
The poll() call is effectively a callback to irqfd_ptable_queue_proc.
So, after the above change, rqfd_wakeup takes irq_srcu inside
wqh->lock, while kvm_irqfd_assign would take them in the opposite order.
However, this is a read-side critical section so this doesn't cause a
deadlock directly. The effect is only that synchronize_srcu would now
wait for wqh->lock to be released. The opposite, which *would* cause a
deadlock, would be a call to synchronize_srcu while wqh->lock is held.
However, this cannot happen because wqh->lock is a spinlock and
synchronize_srcu, which sleeps, cannot be called at all while wqh->lock
is held. So I think it's okay.
>> return 0;
>> + /* irq_srcu is *not* held here. */
>> if (irqfd->resampler)