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

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Message-ID: <aGOoCa3BkV7KrwVz@yilunxu-OptiPlex-7050>
Date: Tue, 1 Jul 2025 17:19:05 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>,
	"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

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@nvidia.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)
                                            ...

We also can't introduce some vdev side lock instead of idev side lock,
vdev could also be freed right after iommufd_vdevice_destroy().

I think the only simple way is to let idev destruction wait for vdev
destruction, that go back to the wait_event idea ...

Thanks,
Yilun