linux-kernel - RE: [PATCH v2 3/4] iommufd: Destroy vdevice on idevice destroy

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Message-ID: <BN9PR11MB52762BD911A3CE2F1A062B158C78A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Tue, 24 Jun 2025 23:57:31 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>, Xu Yilun <yilun.xu@...ux.intel.com>
CC: "will@...nel.org" <will@...nel.org>, "aneesh.kumar@...nel.org"
	<aneesh.kumar@...nel.org>, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"joro@...tes.org" <joro@...tes.org>, "robin.murphy@....com"
	<robin.murphy@....com>, "shuah@...nel.org" <shuah@...nel.org>,
	"nicolinc@...dia.com" <nicolinc@...dia.com>, "aik@....com" <aik@....com>,
	"Williams, Dan J" <dan.j.williams@...el.com>, "baolu.lu@...ux.intel.com"
	<baolu.lu@...ux.intel.com>, "Xu, Yilun" <yilun.xu@...el.com>
Subject: RE: [PATCH v2 3/4] iommufd: Destroy vdevice on idevice destroy

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, June 24, 2025 10:54 PM
> 
> On Mon, Jun 23, 2025 at 05:49:45PM +0800, Xu Yilun wrote:
> > +static void iommufd_device_remove_vdev(struct iommufd_device *idev)
> > +{
> > +	bool vdev_removing = false;
> > +
> > +	mutex_lock(&idev->igroup->lock);
> > +	if (idev->vdev) {
> > +		struct iommufd_vdevice *vdev;
> > +
> > +		vdev = iommufd_get_vdevice(idev->ictx, idev->vdev->obj.id);
> > +		if (IS_ERR(vdev)) {
> 
> This incrs obj.users which will cause a concurrent
> iommufd_object_remove() to fail with -EBUSY, which we are trying to
> avoid.

concurrent remove means a user-initiated IOMMU_DESTROY, for which 
failing with -EBUSY is expected as it doesn't wait for shortterm?

> 
> Also you can hit a race where the tombstone has NULL'd the entry but
> the racing destroy will then load the NULL with xas_load() and hit this:
> 
> 		if (WARN_ON(obj != to_destroy)) {

IOMMU_DESTROY doesn't provide to_destroy.

or are you concerned about the racing between two kernel-initiated
destroy paths? at least for vdev story this doesn't sound to be the
case...