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

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Message-ID: <20250701121315.GD167785@nvidia.com>
Date: Tue, 1 Jul 2025 09:13:15 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Xu Yilun <yilun.xu@...ux.intel.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 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@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)
>                                             ...

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??

Jason