Gleb Natapov wrote: > On Mon, Jul 13, 2009 at 09:26:21AM -0400, Gregory Haskins wrote: > >> Gleb Natapov wrote: >> >>> On Mon, Jul 13, 2009 at 04:02:56PM +0300, Michael S. Tsirkin wrote: >>> >>> >>>> On Sun, Jul 12, 2009 at 03:03:53PM +0300, Gleb Natapov wrote: >>>> >>>> >>>>> Use RCU locking for mask/ack notifiers lists. >>>>> >>>>> Signed-off-by: Gleb Natapov <gleb@xxxxxxxxxx> >>>>> --- >>>>> virt/kvm/irq_comm.c | 20 +++++++++++--------- >>>>> 1 files changed, 11 insertions(+), 9 deletions(-) >>>>> >>>>> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c >>>>> index 5dde1ef..ba3a115 100644 >>>>> --- a/virt/kvm/irq_comm.c >>>>> +++ b/virt/kvm/irq_comm.c >>>>> @@ -179,18 +179,18 @@ void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin) >>>>> break; >>>>> } >>>>> } >>>>> - rcu_read_unlock(); >>>>> >>>>> - hlist_for_each_entry(kian, n, &kvm->irq_ack_notifier_list, link) >>>>> + hlist_for_each_entry_rcu(kian, n, &kvm->irq_ack_notifier_list, link) >>>>> if (kian->gsi == gsi) >>>>> kian->irq_acked(kian); >>>>> + rcu_read_unlock(); >>>>> } >>>>> >>>>> void kvm_register_irq_ack_notifier(struct kvm *kvm, >>>>> struct kvm_irq_ack_notifier *kian) >>>>> { >>>>> mutex_lock(&kvm->irq_lock); >>>>> - hlist_add_head(&kian->link, &kvm->irq_ack_notifier_list); >>>>> + hlist_add_head_rcu(&kian->link, &kvm->irq_ack_notifier_list); >>>>> mutex_unlock(&kvm->irq_lock); >>>>> } >>>>> >>>>> @@ -198,8 +198,9 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm, >>>>> struct kvm_irq_ack_notifier *kian) >>>>> { >>>>> mutex_lock(&kvm->irq_lock); >>>>> - hlist_del_init(&kian->link); >>>>> + hlist_del_init_rcu(&kian->link); >>>>> mutex_unlock(&kvm->irq_lock); >>>>> + synchronize_rcu(); >>>>> } >>>>> >>>>> >>>> This is done under kvm->lock still, which means the lock might be held >>>> potentially for a very long time. Can synchronize_rcu be moved out of >>>> this lock? >>>> >>>> >>>> >>> Only if kvm_free_assigned_device() will be moved out of this lock. >>> Device de-assignment is not very frequent event though. How long do you >>> think it may be held? KVM RCU read sections are very brief. >>> >>> >> Note that the delay imposed by the barrier is not only related to the >> length of the critical section. The barrier blocks until the next grace >> period, and depending on the type of RCU you are using and your config >> options, this could be multiple milliseconds. >> >> I am not saying that this is definitely a problem for your design. I >> am just pointing out that the length of the KVM-RCU read section is only >> > Yeah I understand that other RCU read section may introduce delays too. > The question is how big the delay may be. I think you are misunderstanding me. The read-side CS is not a significant factor here so I am not worried about concurrent read-side CS causing a longer delay. What I am saying is that the grace period of your RCU subsystem is the dominant factor in the equation here, and this may be several milliseconds. > I don't think multiple > milliseconds delay in device de-assignment is a big issue though. > I would tend to agree with you. It's not fast path. I only brought this up because I saw your design being justified incorrectly: you said "KVM RCU read sections are very brief", but that is not really relevant to Michael's point. I just want to make sure that the true impact is understood. Kind Regards, -Greg
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