Re: [PATCH 4/4] Convert irq notifiers lists to RCU locking.

[Gleb Natapov <gleb@redhat.com>]

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@redhat.com>
> > ---
> >  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.

> >  int kvm_request_irq_source_id(struct kvm *kvm)
> > @@ -246,7 +247,7 @@ void kvm_register_irq_mask_notifier(struct kvm *kvm, int irq,
> >  {
> >  	mutex_lock(&kvm->irq_lock);
> >  	kimn->irq = irq;
> > -	hlist_add_head(&kimn->link, &kvm->mask_notifier_list);
> > +	hlist_add_head_rcu(&kimn->link, &kvm->mask_notifier_list);
> >  	mutex_unlock(&kvm->irq_lock);
> >  }
> >  
> > @@ -254,8 +255,9 @@ void kvm_unregister_irq_mask_notifier(struct kvm *kvm, int irq,
> >  				      struct kvm_irq_mask_notifier *kimn)
> >  {
> >  	mutex_lock(&kvm->irq_lock);
> > -	hlist_del(&kimn->link);
> > +	hlist_del_rcu(&kimn->link);
> >  	mutex_unlock(&kvm->irq_lock);
> > +	synchronize_rcu();
> >  }
> >  
> >  void kvm_fire_mask_notifiers(struct kvm *kvm, int irq, bool mask)
> > @@ -263,11 +265,11 @@ void kvm_fire_mask_notifiers(struct kvm *kvm, int irq, bool mask)
> >  	struct kvm_irq_mask_notifier *kimn;
> >  	struct hlist_node *n;
> >  
> > -	WARN_ON(!mutex_is_locked(&kvm->irq_lock));
> > -
> > -	hlist_for_each_entry(kimn, n, &kvm->mask_notifier_list, link)
> > +	rcu_read_lock();
> > +	hlist_for_each_entry_rcu(kimn, n, &kvm->mask_notifier_list, link)
> >  		if (kimn->irq == irq)
> >  			kimn->func(kimn, mask);
> > +	rcu_read_unlock();
> >  }
> >  
> >  void kvm_free_irq_routing(struct kvm *kvm)
> > -- 
> > 1.6.2.1
> > 
> > --
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--
			Gleb.