Re: [PATCH 1/1] KVM: Fix potentially recursively get kvm lock

Yang, Sheng
2009-05-12T14:12:52Z

On Tuesday 12 May 2009 19:55:24 Marcelo Tosatti wrote:
> On Tue, May 12, 2009 at 05:32:09PM +0800, Sheng Yang wrote:
> > kvm_vm_ioctl_deassign_dev_irq() would potentially recursively get
> > kvm->lock, because it called kvm_deassigned_irq() which implicit hold
> > kvm->lock by calling deassign_host_irq().
> >
> > Fix it by move kvm_deassign_irq() out of critial region. And add the
> > missing lock for deassign_guest_irq().
> >
> > Reported-by: Alex Williamson <alex.williamson@hp.com>
> > Signed-off-by: Sheng Yang <sheng@linux.intel.com>
> > ---
> > virt/kvm/kvm_main.c | 14 +++++++-------
> > 1 files changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 4d00942..3c69655 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -215,6 +215,8 @@ static void kvm_assigned_dev_ack_irq(struct
> > kvm_irq_ack_notifier *kian) static void deassign_guest_irq(struct kvm
> > *kvm,
> > struct kvm_assigned_dev_kernel *assigned_dev)
> > {
> > + mutex_lock(&kvm->lock);
> > +
> > kvm_unregister_irq_ack_notifier(&assigned_dev->ack_notifier);
> > assigned_dev->ack_notifier.gsi = -1;
> >
> > @@ -222,6 +224,8 @@ static void deassign_guest_irq(struct kvm *kvm,
> > kvm_free_irq_source_id(kvm, assigned_dev->irq_source_id);
> > assigned_dev->irq_source_id = -1;
> > assigned_dev->irq_requested_type &= ~(KVM_DEV_IRQ_GUEST_MASK);
> > +
> > + mutex_unlock(&kvm->lock);
> > }
> >
> > /* The function implicit hold kvm->lock mutex due to cancel_work_sync()
> > */ @@ -558,20 +562,16 @@ static int kvm_vm_ioctl_deassign_dev_irq(struct
> > kvm *kvm, struct kvm_assigned_irq
> > *assigned_irq)
> > {
> > - int r = -ENODEV;
> > struct kvm_assigned_dev_kernel *match;
> >
> > mutex_lock(&kvm->lock);
> > -
> > match = kvm_find_assigned_dev(&kvm->arch.assigned_dev_head,
> > assigned_irq->assigned_dev_id);
> > + mutex_unlock(&kvm->lock);
>
> assigned_dev list is protected by kvm->lock. So you could have another
> ioctl adding to it at the same time you're searching.

Oh, yes... My fault...

> Could either have a separate kvm->assigned_devs_lock, to protect
> kvm->arch.assigned_dev_head (users are ioctls that manipulate it), or
> change the IRQ injection to use a separate spinlock, kill the workqueue
> and call kvm_set_irq from the assigned device interrupt handler.

Peferred the latter, though needs more work. But the only reason for put a
workqueue here is because kvm->lock is a mutex? I can't believe... If so, I
think we had made a big mistake - we have to fix all kinds of racy problem
caused by this, but finally find it's unnecessary...

Maybe another reason is kvm_kick_vcpu(), but have already fixed by you.

Continue to check the code...

--
regards
Yang, Sheng