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

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2022/5/5 21:38, Jean-Philippe Brucker wrote:
> 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

Ah! Yes. Thank you for the clarification.

> 
> 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.
>>>           */

Agreed. The mutex is needed only when domain could possibly be freed
before unbind(). In that case, we need this mutex and get a reference
from the domain. As we have dropped the domain user reference, this lock
is unnecessary.

>>>
>>> Thanks,
>>> Jean
>>>
>>>> +	domain = xa_load(&group->pasid_array, pasid);
>>>> +	mutex_unlock(&group->mutex);
>>>> +	iommu_group_put(group);
>>>> +
>>>> +	return domain;
>>>> +}
>>

Best regards,
baolu