Re: [PATCH v5 10/12] iommu: Prepare IOMMU domain for IOPF

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

Hi Baolu,

On Thu, May 05, 2022 at 04:31:38PM +0800, Baolu Lu wrote:
> On 2022/5/4 02:20, Jean-Philippe Brucker wrote:
> > > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > > index 7cae631c1baa..33449523afbe 100644
> > > --- a/drivers/iommu/iommu.c
> > > +++ b/drivers/iommu/iommu.c
> > > @@ -3174,3 +3174,24 @@ void iommu_detach_device_pasid(struct iommu_domain *domain,
> > >   	iommu_group_put(group);
> > >   }
> > > +
> > > +struct iommu_domain *iommu_get_domain_for_dev_pasid(struct device *dev,
> > > +						    ioasid_t pasid)
> > > +{
> > > +	struct iommu_domain *domain;
> > > +	struct iommu_group *group;
> > > +
> > > +	if (!pasid_valid(pasid))
> > > +		return NULL;
> > > +
> > > +	group = iommu_group_get(dev);
> > > +	if (!group)
> > > +		return NULL;
> > > +
> > > +	mutex_lock(&group->mutex);
> > Unfortunately this still causes the deadlock when unbind() flushes the
> > IOPF queue while holding the group mutex.
> 
> Sorry, I didn't get your point here.
> 
> Do you mean unbind() could hold group mutex before calling this helper?
> The group mutex is only available in iommu.c. The unbind() has no means
> to hold this lock. Or, I missed anything?

I wasn't clear, it's iommu_detach_device_pasid() that holds the
group->mutex:

 iommu_sva_unbind_device()          |
  iommu_detach_device_pasid()       |
   mutex_lock(&group->mutex)        |
   domain->ops->detach_dev_pasid()  | iopf_handle_group()
    iopf_queue_flush_dev()          |  iommu_get_domain_for_dev_pasid()
     ... wait for IOPF work         |   mutex_lock(&group->mutex)
                                    |    ... deadlock

Thanks,
Jean

> 
> Best regards,
> baolu
> 
> > 
> > If we make this function private to IOPF, then we can get rid of this
> > mutex_lock(). It's OK because:
> > 
> > * xarray protects its internal state with RCU, so we can call
> >    xa_load() outside the lock.
> > 
> > * The domain obtained from xa_load is finalized. Its content is valid
> >    because xarray stores the domain using rcu_assign_pointer(), which has a
> >    release memory barrier, which pairs with data dependencies in IOPF
> >    (domain->sva_ioas etc).
> > 
> >    We'll need to be careful about this when allowing other users to install
> >    a fault handler. Should be fine as long as the handler and data are
> >    installed before the domain is added to pasid_array.
> > 
> > * We know the domain is valid the whole time IOPF is using it, because
> >    unbind() waits for pending faults.
> > 
> > We just need a comment explaining the last point, something like:
> > 
> >         /*
> > 	* Safe to fetch outside the group mutex because:
> >          * - xarray protects its internal state with RCU
> >          * - the domain obtained is either NULL or fully formed
> > 	* - the IOPF work is the only caller and is flushed before the
> > 	*   domain is freed.
> >          */
> > 
> > Thanks,
> > Jean
> > 
> > > +	domain = xa_load(&group->pasid_array, pasid);
> > > +	mutex_unlock(&group->mutex);
> > > +	iommu_group_put(group);
> > > +
> > > +	return domain;
> > > +}
> 
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