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

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Message-ID: <ZyPQhn+DlT4Zy3Qq@Asurada-Nvidia>
Date: Thu, 31 Oct 2024 11:46:30 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...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 02:04:46PM -0300, Jason Gunthorpe wrote:
> 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

Ack. I'll still update this since xa_cmpxchg can give other errno:
+	/* xa_cmpxchg is okay to fail if alloc returned -EEXIST previously */
-	/* xa_cmpxchg is okay to fail if alloc failed xa_cmpxchg previously */

Thanks
Nicolin