On 19/12/2017 07:35, Lan Tianyu wrote: > On 2017年12月18日 16:50, 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 -> >>> unlock()) >> >> 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); >> } >> #endif >> + srcu_read_unlock(&kvm->irq_srcu, idx); >> >> return 0; >> >> fail: >> + /* irq_srcu is *not* held here. */ >> if (irqfd->resampler) >> irqfd_resampler_shutdown(irqfd); >> >> > > Hi Paolo: > This patch still can't prevent from freeing struct irqfd in > irq_shutdown() during assign irqfd. Once irqfd is added to > kvm->irqfds.items list, deassign path can get irqfd and free it. You're right, that also has to be protected by SRCU. So a new bool is needed (probably "active" more than "initialized") in order to replace list_del_init with list_del_rcu. Paolo
