linux-kernel - Re: [PATCH v5 09/13] iommufd/selftest: Add refcount to mock_iommu

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Message-ID: <20241029185322.GX6956@nvidia.com>
Date: Tue, 29 Oct 2024 15:53:22 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, 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 v5 09/13] iommufd/selftest: Add refcount to
 mock_iommu_device

On Tue, Oct 29, 2024 at 09:02:58AM -0700, Nicolin Chen wrote:
> On Tue, Oct 29, 2024 at 12:34:38PM -0300, Jason Gunthorpe wrote:
> > On Fri, Oct 25, 2024 at 04:49:49PM -0700, Nicolin Chen wrote:
> > > For an iommu_dev that can unplug (so far only this selftest does so), the
> > > viommu->iommu_dev pointer has no guarantee of its life cycle after it is
> > > copied from the idev->dev->iommu->iommu_dev.
> > > 
> > > Track the user count of the iommu_dev. Postpone the exit routine using a
> > > completion, if refcount is unbalanced. The refcount inc/dec will be added
> > > in the following patch.
> > > 
> > > Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> > > ---
> > >  drivers/iommu/iommufd/selftest.c | 32 ++++++++++++++++++++++++--------
> > >  1 file changed, 24 insertions(+), 8 deletions(-)
> > 
> > Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> > 
> > Since this is built into the iommufd module it can't be unloaded
> > without also unloading iommufd, which is impossible as long as any
> > iommufd FDs are open. So I expect that the WARN_ON can never happen.
> 
> Hmm, I assume we still need this patch then?

I was thinking, I think it still is a reasonable example of what it
might look like

You might include the above remark as a comment above the WARN_ON though.

> Could a faulty "--force" possibly trigger it?

I'm not sure, I suspect not?

Jason