Re: [PATCH v4 03/26] iommu: Add a page fault handler

[Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>]

On Wed, Feb 26, 2020 at 01:59:33PM +0000, Jonathan Cameron wrote:
> > +static int iopf_complete(struct device *dev, struct iopf_fault *iopf,
> > +			 enum iommu_page_response_code status)
> 
> This is called once per group.  Should name reflect that?

Ok

[...]
> > +/**
> > + * iommu_queue_iopf - IO Page Fault handler
> > + * @evt: fault event
> > + * @cookie: struct device, passed to iommu_register_device_fault_handler.
> > + *
> > + * Add a fault to the device workqueue, to be handled by mm.
> > + *
> > + * Return: 0 on success and <0 on error.
> > + */
> > +int iommu_queue_iopf(struct iommu_fault *fault, void *cookie)
> > +{
> > +	int ret;
> > +	struct iopf_group *group;
> > +	struct iopf_fault *iopf, *next;
> > +	struct iopf_device_param *iopf_param;
> > +
> > +	struct device *dev = cookie;
> > +	struct iommu_param *param = dev->iommu_param;
> > +
> > +	if (WARN_ON(!mutex_is_locked(&param->lock)))
> > +		return -EINVAL;
> 
> Just curious...
> 
> Why do we always need a runtime check on this rather than say,
> using lockdep_assert_held or similar?

I probably didn't know about lockdep_assert at the time :)

> > +	/*
> > +	 * It is incredibly easy to find ourselves in a deadlock situation if
> > +	 * we're not careful, because we're taking the opposite path as
> > +	 * iommu_queue_iopf:
> > +	 *
> > +	 *   iopf_queue_flush_dev()   |  PRI queue handler
> > +	 *    lock(&param->lock)      |   iommu_queue_iopf()
> > +	 *     queue->flush()         |    lock(&param->lock)
> > +	 *      wait PRI queue empty  |
> > +	 *
> > +	 * So we can't hold the device param lock while flushing. Take a
> > +	 * reference to the device param instead, to prevent the queue from
> > +	 * going away.
> > +	 */
> > +	mutex_lock(&param->lock);
> > +	iopf_param = param->iopf_param;
> > +	if (iopf_param) {
> > +		queue = param->iopf_param->queue;
> > +		iopf_param->busy = true;
> 
> Describing this as taking a reference is not great...
> I'd change the comment to set a flag or something like that.
> 
> Is there any potential of multiple copies of this running against
> each other?  I've not totally gotten my head around when this
> might be called yet.

Yes it's allowed, this should be a refcount

[...]
> > +int iopf_queue_remove_device(struct iopf_queue *queue, struct device *dev)
> > +{
> > +	int ret = -EINVAL;
> > +	struct iopf_fault *iopf, *next;
> > +	struct iopf_device_param *iopf_param;
> > +	struct iommu_param *param = dev->iommu_param;
> > +
> > +	if (!param || !queue)
> > +		return -EINVAL;
> > +
> > +	do {
> > +		mutex_lock(&queue->lock);
> > +		mutex_lock(&param->lock);
> > +		iopf_param = param->iopf_param;
> > +		if (iopf_param && iopf_param->queue == queue) {
> > +			if (iopf_param->busy) {
> > +				ret = -EBUSY;
> > +			} else {
> > +				list_del(&iopf_param->queue_list);
> > +				param->iopf_param = NULL;
> > +				ret = 0;
> > +			}
> > +		}
> > +		mutex_unlock(&param->lock);
> > +		mutex_unlock(&queue->lock);
> > +
> > +		/*
> > +		 * If there is an ongoing flush, wait for it to complete and
> > +		 * then retry. iopf_param isn't going away since we're the only
> > +		 * thread that can free it.
> > +		 */
> > +		if (ret == -EBUSY)
> > +			wait_event(iopf_param->wq_head, !iopf_param->busy);
> > +		else if (ret)
> > +			return ret;
> > +	} while (ret == -EBUSY);
> 
> I'm in two minds about the next comment (so up to you)...
> 
> Currently this looks a bit odd.  Would you be better off just having a separate
> parameter for busy and explicit separate handling for the error path?
> 
> 	bool busy;
> 	int ret = 0;
> 
> 	do {
> 		mutex_lock(&queue->lock);
> 		mutex_lock(&param->lock);
> 		iopf_param = param->iopf_param;
> 		if (iopf_param && iopf_param->queue == queue) {
> 			busy = iopf_param->busy;
> 			if (!busy) {
> 				list_del(&iopf_param->queue_list);
> 				param->iopf_param = NULL;
> 			}
> 		} else {
> 			ret = -EINVAL;
> 		}
> 		mutex_unlock(&param->lock);
> 		mutex_unlock(&queue->lock);
> 		if (ret)
> 			return ret;
> 		if (busy)
> 			wait_event(iopf_param->wq_head, !iopf_param->busy);
> 		
> 	} while (busy);
> 
> 	..

Sure, I think it looks better

Thanks,
Jean