RE: [PATCH v2 07/10] iommufd/device: Make hwpt_list list_add/del symmetric

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Thursday, February 9, 2023 9:24 PM
> 
> On Thu, Feb 09, 2023 at 03:23:47AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > > Sent: Wednesday, February 8, 2023 5:18 AM
> > >
> > > Because list_del() is together with iopt_table_remove_domain(), it makes
> > > sense to have list_add_tail() together with iopt_table_add_domain().
> > >
> > > Also place the mutex outside the iommufd_device_do_attach() call,
> similar
> > > to what's in the iommufd_device_auto_get_domain() function.
> > >
> > > Co-developed-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> > > Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> >
> > shouldn't this be a separate bug fix and backported? double adding a
> > list item would certainly clobber the list...
> 
> AFAIK there is no bug, this is just reorganizing things
> 

there is semantics change.

here is the current code:

	case IOMMUFD_OBJ_HW_PAGETABLE: {
		struct iommufd_hw_pagetable *hwpt =
			container_of(pt_obj, struct iommufd_hw_pagetable, obj);

		rc = iommufd_device_do_attach(idev, hwpt);
		if (rc)
			goto out_put_pt_obj;

		mutex_lock(&hwpt->ioas->mutex);
		list_add_tail(&hwpt->hwpt_item, &hwpt->ioas->hwpt_list);
		mutex_unlock(&hwpt->ioas->mutex);
		break;
	}

Above means every attach to hwpt will try to add the hwpt to the
list tail. Isn't it a bug?

with this patch the hwpt is added to the list only when it's attached
by the first device, i.e. when iopt_table_add_domain() is called.

	if (!iommufd_hw_pagetable_has_group(hwpt, idev->group)) {
		rc = iommu_attach_group(hwpt->domain, idev->group);
		if (rc)
			goto out_iova;

		if (list_empty(&hwpt->devices)) {
			rc = iopt_table_add_domain(&hwpt->ioas->iopt,
						hwpt->domain);
			if (rc)
				goto out_detach;
			list_add_tail(&hwpt->hwpt_item, &hwpt->ioas->hwpt_list);
		}
	}

so it's actually a bug fix.