Date:   Thu, 6 Jan 2022 21:19:29 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Lu Baolu <baolu.lu@linux.intel.com>
Cc:     Joerg Roedel <joro@8bytes.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@infradead.org>,
        Kevin Tian <kevin.tian@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Will Deacon <will@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>, rafael@kernel.org,
        Diana Craciun <diana.craciun@oss.nxp.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Eric Auger <eric.auger@redhat.com>,
        Liu Yi L <yi.l.liu@intel.com>,
        Jacob jun Pan <jacob.jun.pan@intel.com>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Stuart Yoder <stuyoder@gmail.com>,
        Laurentiu Tudor <laurentiu.tudor@nxp.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Li Yang <leoyang.li@nxp.com>,
        Dmitry Osipenko <digetx@gmail.com>,
        iommu@lists.linux-foundation.org, linux-pci@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach_device() for
 multi-device groups
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On Fri, Jan 07, 2022 at 09:14:38AM +0800, Lu Baolu wrote:

> > Once we know our calling context we can always automatic switch from
> > DMA API mode to another domain without any trouble or special
> > counters:
> > 
> > if (!dev->driver->no_kernel_api_dma) {
> >      if (group->owner_cnt > 1 || group->owner)
> >          return -EBUSY;
> >      return __iommu_attach_group(domain, group);
> > }
> 
> Is there any lock issue when referencing dev->driver here? I guess this
> requires iommu_attach_device() only being called during the driver life
> (a.k.a. between driver .probe and .release).

Yes, that is correct. That would need to be documented.

It is the same reason the routine was able to get the group from the
dev. The dev's group must be stable so long as a driver is attached or
everything is broken :)

Much of the group refcounting code is useless for this reason. The
group simply cannot be concurrently destroyed in these contexts.

Jason