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

["Yang, Sheng" <sheng.yang@intel.com>]

On Wednesday 13 May 2009 06:09:08 Marcelo Tosatti wrote:
> On Tue, May 12, 2009 at 03:36:27PM -0600, Alex Williamson wrote:
> > On Tue, 2009-05-12 at 16:44 -0300, Marcelo Tosatti wrote:
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index 4d00942..ba067db 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -250,7 +250,15 @@ static void deassign_host_irq(struct kvm *kvm,
> > >  			disable_irq_nosync(assigned_dev->
> > >  					   host_msix_entries[i].vector);
> > >
> > > +		/*
> > > +		 * FIXME: kvm_assigned_dev_interrupt_work_handler can deadlock
> > > +		 * with cancel_work_sync, since it requires kvm->lock for irq
> > > +		 * injection. This is a hack, the irq code must use
> > > +		 * a separate lock.
> > > +		 */
> > > +		mutex_unlock(&kvm->lock);
> > >  		cancel_work_sync(&assigned_dev->interrupt_work);
> > > +		mutex_lock(&kvm->lock);
> >
> > Seems to work, I assume you've got a similar unlock/lock for the
> > MSI/INTx block.  Thanks,
>
> KVM: workaround workqueue / deassign_host_irq deadlock
>
> I think I'm running into the following deadlock in the kvm kernel module
> when trying to use device assignment:
>
> CPU A                               CPU B
> kvm_vm_ioctl_deassign_dev_irq()
>   mutex_lock(&kvm->lock);           worker_thread()
>   -> kvm_deassign_irq()               ->
> kvm_assigned_dev_interrupt_work_handler()
>     -> deassign_host_irq()              mutex_lock(&kvm->lock);
>       -> cancel_work_sync() [blocked]
>
> Workaround the issue by dropping kvm->lock for cancel_work_sync().
>
> Reported-by: Alex Williamson <alex.williamson@hp.com>
> From: Sheng Yang <sheng.yang@intel.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Another calling path(kvm_free_all_assigned_devices()) don't hold kvm->lock... 
Seems it need the lock for travel assigned dev list?

-- 
regards
Yang, Sheng

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 4d00942..d4af719 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -250,7 +250,15 @@ static void deassign_host_irq(struct kvm *kvm,
>  			disable_irq_nosync(assigned_dev->
>  					   host_msix_entries[i].vector);
>
> +		/*
> +		 * FIXME: kvm_assigned_dev_interrupt_work_handler can deadlock
> +		 * with cancel_work_sync, since it requires kvm->lock for irq
> +		 * injection. This is a hack, the irq code must use
> +		 * a separate lock. Same below for MSI.
> +		 */
> +		mutex_unlock(&kvm->lock);
>  		cancel_work_sync(&assigned_dev->interrupt_work);
> +		mutex_lock(&kvm->lock);
>
>  		for (i = 0; i < assigned_dev->entries_nr; i++)
>  			free_irq(assigned_dev->host_msix_entries[i].vector,
> @@ -263,7 +271,9 @@ static void deassign_host_irq(struct kvm *kvm,
>  	} else {
>  		/* Deal with MSI and INTx */
>  		disable_irq_nosync(assigned_dev->host_irq);
> +		mutex_unlock(&kvm->lock);
>  		cancel_work_sync(&assigned_dev->interrupt_work);
> +		mutex_lock(&kvm->lock);
>
>  		free_irq(assigned_dev->host_irq, (void *)assigned_dev);