linux-kernel - Re: [PATCH v6 01/10] iommufd/viommu: Add IOMMUFD_OBJ_VDEVICE and IOMMU_VDEVICE

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Message-ID: <20241031170446.GQ10193@nvidia.com>
Date: Thu, 31 Oct 2024 14:04:46 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, joro@...tes.org,
	suravee.suthikulpanit@....com, will@...nel.org,
	robin.murphy@....com, dwmw2@...radead.org, shuah@...nel.org,
	iommu@...ts.linux.dev, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	baolu.lu@...ux.intel.com, eric.auger@...hat.com,
	jean-philippe@...aro.org, mdf@...nel.org, mshavit@...gle.com,
	shameerali.kolothum.thodi@...wei.com, smostafa@...gle.com,
	yi.l.liu@...el.com, aik@....com, zhangfei.gao@...aro.org,
	patches@...ts.linux.dev
Subject: Re: [PATCH v6 01/10] iommufd/viommu: Add IOMMUFD_OBJ_VDEVICE and
 IOMMU_VDEVICE_ALLOC ioctl

On Thu, Oct 31, 2024 at 09:56:37AM -0700, Nicolin Chen wrote:
> On Thu, Oct 31, 2024 at 10:29:41AM -0300, Jason Gunthorpe wrote:
> > On Wed, Oct 30, 2024 at 02:35:27PM -0700, Nicolin Chen wrote:
> > > +void iommufd_vdevice_destroy(struct iommufd_object *obj)
> > > +{
> > > +	struct iommufd_vdevice *vdev =
> > > +		container_of(obj, struct iommufd_vdevice, obj);
> > > +	struct iommufd_viommu *viommu = vdev->viommu;
> > > +
> > > +	/* xa_cmpxchg is okay to fail if alloc returned -EEXIST previously */
> > > +	xa_cmpxchg(&viommu->vdevs, vdev->id, vdev, NULL, GFP_KERNEL);
> > 
> > There are crazy races that would cause this not to work. Another
> > thread could have successfully destroyed whatever caused EEXIST and
> > the successfully registered this same vdev to the same id. Then this
> > will wrongly erase the other threads entry.
> >
> > It would be better to skip the erase directly if the EEXIST unwind is
> > being taken.
>
> Hmm, is the "another thread" an alloc() or a destroy()? 

I was thinking both

> It doesn't seem to me that there could be another destroy() on the
> same object since this current destroy() is the abort to an
> unfinalized object.  And it doesn't seem that another alloc() will
> get the same vdev ptr since every vdev allocation in the alloc()
> will be different?

Ah so you are saying that since the vdev 'old' is local to this thread
it can't possibly by aliased by another?

I was worried the id could be aliased, but yes, that seems right that
the vdev cmpxchg would reject that.

So lets leave it

Jason