linux-kernel - Re: [PATCH] kvm: lock slots_lock around device assignment

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Date:	Wed, 18 Apr 2012 20:58:24 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jbaron@...hat.com, jan.kiszka@...mens.com
Subject: Re: [PATCH] kvm: lock slots_lock around device assignment

On Wed, 2012-04-18 at 23:30 -0300, Marcelo Tosatti wrote:
> On Tue, Apr 17, 2012 at 09:46:44PM -0600, Alex Williamson wrote:
> > @@ -340,7 +343,11 @@ int kvm_iommu_unmap_guest(struct kvm *kvm)
> >  	if (!domain)
> >  		return 0;
> >  
> > +	mutex_lock(&kvm->slots_lock);
> >  	kvm_iommu_unmap_memslots(kvm);
> > +	kvm->arch.iommu_domain = NULL;
> > +	mutex_unlock(&kvm->slots_lock);
> > +
> >  	iommu_domain_free(domain);
> >  	return 0;
> >  }
> 
> This might trigger lockdep warnings due to
> 
> kvm_vm_ioctl_create_vcpu
> mutex_lock(&kvm->lock)
> kvm_put_kvm(kvm)
> kvm_destroy_vm
> kvm_iommu_unmap_guest 
> 
> sequence.
> 
> Better drop it, it is not necessary in vm destruction
> path (since only user is self).

I actually ran this with lockdep and didn't generate a warning;
hopefully I had it configured correctly.  Also, we'll soon be unmapping
the guest any time we remove the last assigned device so this will no
longer be a vm destruction-only path.  We can just as easily race adding
new mappings or removing already removed ones on that path.  We also
acquire kvm->lock in the mapping path:

kvm_vm_ioctl_assign_device() {
mutex_lock(&kvm->lock);
if (!kvm->arch.iommu_domain) {
      r = kvm_iommu_map_guest(kvm);

which by inspection and the lock ordering note in kvm_main seems to be
ok.  Thanks,

Alex

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