Re: [RFC PATCH 1/7] iommu: Add group lookup by ID

[Jason Gunthorpe]

On Tue, Jul 14, 2026 at 09:06:51PM +0800, Zhanpeng Zhang wrote:
> Add iommu_group_get_by_id() so callers can resolve an IOMMU group from
> the numeric ID used in /sys/kernel/iommu_groups.
> 
> An ID lookup must keep the group object alive without also keeping an
> otherwise empty group active. Embed the devices kobject in struct
> iommu_group so its address remains valid until the parent group is
> released, and return a reference on the parent kobject to ID lookup
> callers. Add iommu_group_put_by_id() to release that reference and
> iommu_group_is_active() to detect when the devices kobject has become
> inactive.
> 
> Serialize lookup against group teardown with iommu_group_kset_mutex and
> only return groups whose devices kobject still has a live reference.
> This prevents a concurrent lookup from dereferencing a stale child
> kobject while allowing external users to discard bindings to empty
> groups.
> 
> Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper@xxxxxxxxxxxxx>
> ---
>  drivers/iommu/iommu.c | 107 +++++++++++++++++++++++++++++++++++++-----
>  include/linux/iommu.h |  17 +++++++
>  2 files changed, 113 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index e8f13dcebbde..da269d10f6bf 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -40,6 +40,7 @@
>  #include "iommu-priv.h"
>  
>  static struct kset *iommu_group_kset;
> +static DEFINE_MUTEX(iommu_group_kset_mutex);
>  static DEFINE_IDA(iommu_group_ida);

I think it would be better to change the ida to an xarray than to use
a string search on a kset..

Jason