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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, May 13, 2009 at 10:07:54AM +0800, Yang, Sheng wrote:
> > 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?

Sheng,

The task executing the deassign irq ioctl has a reference to the vm
instance. This solution is just temporary though until the locks can be
split and then dropping kvm->lock around cancel_work_sync will not be
necessary anymore.