linux-kernel - RE: [PATCH v3 2/5] iommufd: Destroy vdevice on idevice destroy

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Message-ID: <BN9PR11MB52769790C63FFCDC711E80DB8C40A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 2 Jul 2025 09:13:50 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>, Jason Gunthorpe <jgg@...dia.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 v3 2/5] iommufd: Destroy vdevice on idevice destroy

> From: Xu Yilun <yilun.xu@...ux.intel.com>
> Sent: Wednesday, July 2, 2025 10:24 AM
> 
> On Tue, Jul 01, 2025 at 09:13:15AM -0300, Jason Gunthorpe wrote:
> > On Tue, Jul 01, 2025 at 05:19:05PM +0800, Xu Yilun wrote:
> > > On Mon, Jun 30, 2025 at 11:50:51AM -0300, Jason Gunthorpe wrote:
> > > > On Mon, Jun 30, 2025 at 06:18:50PM +0800, Xu Yilun wrote:
> > > >
> > > > > I need to reconsider this, seems we need a dedicated vdev lock to
> > > > > synchronize concurrent vdev abort/destroy.
> > > >
> > > > It is not possible to be concurrent
> > > >
> > > > destroy is only called once after it is no longer possible to call
> > > > abort.
> > >
> > > I'm almost about to drop the "abort twice" idea. [1]
> > >
> > > [1]: https://lore.kernel.org/linux-
> iommu/20250625123832.GF167785@...dia.com/
> > >
> > > See from the flow below,
> > >
> > >   T1. iommufd_device_unbind(idev)
> > > 	iommufd_device_destroy(obj)
> > > 		mutex_lock(&idev->igroup->lock)
> > >  		iommufd_vdevice_abort(idev->vdev.obj)
> > > 		mutex_unlock(&idev->igroup->lock)
> > > 	kfree(obj)
> > >
> > >   T2. iommufd_destroy(vdev_id)
> > > 	iommufd_vdevice_destroy(obj)
> > > 		mutex_lock(&vdev->idev->igroup->lock)
> > > 		iommufd_vdevice_abort(obj);
> > > 		mutex_unlock(&vdev->idev->igroup->lock)
> > > 	kfree(obj)
> > >
> > > iommufd_vdevice_destroy() will access idev->igroup->lock, but it is
> > > possible the idev is already freed at that time:
> > >
> > >                                             iommufd_destroy(vdev_id)
> > >                                             iommufd_vdevice_destroy(obj)
> > >   iommufd_device_unbind(idev)
> > >   iommufd_device_destroy(obj)
> > >   mutex_lock(&idev->igroup->lock)
> > >                                             mutex_lock(&vdev->idev->igroup->lock) (wait)
> > >   iommufd_vdevice_abort(idev->vdev.obj)
> > >   mutex_unlock(&idev->igroup->lock)
> > >   kfree(obj)
> > >                                             mutex_lock(&vdev->idev->igroup->lock) (PANIC)
> > >                                             iommufd_vdevice_abort(obj)
> > >                                             ...
> >
> > Yes, you can't touch idev inside the destroy function at all, under
> > any version. idev is only valid if you have a refcount on vdev.
> >
> > But why are you touching this lock? Arrange things so abort doesn't
> > touch the idev??
> 
> idev has a pointer idev->vdev to track the vdev's lifecycle.
> idev->igroup->lock protects the pointer. At the end of
> iommufd_vdevice_destroy() this pointer should be NULLed so that idev
> knows vdev is really destroyed.
> 
> I haven't found a safer way for vdev to sync up its validness with idev
> w/o touching idev.
> 
> I was thinking of using vdev->idev and some vdev lock for tracking
> instead. Then iommufd_vdevice_abort() doesn't touch idev. But it is
> still the same, just switch to put idev in risk:
> 
> 
>                                                iommufd_destroy(vdev_id)
>                                                iommufd_vdevice_destroy(obj)
>   iommufd_device_unbind(idev)
>   iommufd_device_destroy(obj)
>                                                mutex_lock(&vdev->some_lock)
>   mutex_lock(&idev->vdev->some_lock) (wait)

panic could happen here, between acquiring idev->vdev and
mutex(vdev->some_lock) as vdev might be destroyed/freed 
in-between.

>                                                iommufd_vdevice_abort(obj)
>                                                mutex_unlock(&vdev->some_lock)
>                                                kfree(obj)
>   mutex_lock(&idev->vdev->some_lock) (PANIC)
>   iommufd_vdevice_abort(idev->vdev.obj)
>   ...
> 

I cannot find a safe way either, except using certain global lock.

but comparing to that I'd prefer to the original wait approach...