[PATCH v2 9/10] iommu: Complete the locking for dev->iommu_group

Konrad Dybcio konradybcio at kernel.org
Wed Aug 9 05:55:48 PDT 2023

On 31.07.2023 19:50, Jason Gunthorpe wrote:
> Revise the locking for dev->iommu_group so that it has three safe ways to
> access it:
>  - It is read by a probe'd device driver. So long as a device driver is
>    probed the dev->iommu_group will be guaranteed stable without further
>    locking.
>  - Read under the device_lock(), this primarily protects against
>    parallel probe of the same device, and parallel probe/remove
>  - Read/Write under the global dev_iommu_group_lock. This is used during
>    probe time discovery of groups. Device drivers will scan unlocked
>    portions of the device tree to locate an already existing group. These
>    scans can access the dev->iommu_group under the global lock to single
>    thread determining and installing the group. This ensures that groups
>    are reliably formed.
> Narrow the scope of the global dev_iommu_group_lock to be only during the
> dev->iommu_group setup, and not for the entire probing.
> Prior patches removed the various races inherent to the probe process by
> consolidating all the work under the group->mutex. In this configuration
> it is fine if two devices race to the group_device step of a new
> iommu_group, the group->mutex locking will ensure the group_device and
> domain setup part remains properly ordered.
> Add the missing locking on the remove paths. For iommu_deinit_device() it
> is necessary to hold the dev_iommu_group_lock due to possible races during
> probe error unwind.
> Fully lock the iommu_group_add/remove_device() path so we can use lockdep
> assertions. Other than lockdep this is redundant, VFIO no-iommu doesn't
> use group clustering.
> For iommu_release_device() it is redundant, as we expect no external
> references to the struct device by this point, but it is harmless so
> add the missing lock to allow lockdep assertions to work.
> This resolves the remarks of the comment in __iommu_probe_device().
> Reviewed-by: Lu Baolu <baolu.lu at linux.intel.com>
> Signed-off-by: Jason Gunthorpe <jgg at nvidia.com>
> Reviewed-by: Kevin Tian <kevin.tian at intel.com>
> ---
Hello, this patch breaks booting on at least one Qualcomm platform using
the SMMUv2 driver w/ qcom impl. The board hangs right after SMMU probe.

Reverting it makes the platform boot again.


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