Re: [PATCH v3 05/11] iommu: Change group->devices to RCU-protected list

[Baolu Lu]

On 4/17/26 07:28, Nicolin Chen wrote:
> To allow lockless iterations of the group->devices list in an ISR context
> that cannot hold the group->mutex, change the list to be RCU protected.
>
> Signed-off-by: Nicolin Chen<nicolinc@xxxxxxxxxx>
> ---
>   drivers/iommu/iommu.c | 16 +++++++++-------
>   1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 768ac728b4cc3..d1be62a07904a 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -84,18 +84,20 @@ struct group_device {
>   	 */
>   	bool blocked;
>   	unsigned int reset_depth;
> +	struct rcu_head rcu;
>   };
>   
>   /* Iterate over each struct group_device in a struct iommu_group */
>   #define for_each_group_device(group, pos) \
> -	list_for_each_entry(pos, &(group)->devices, list)
> +	list_for_each_entry_rcu(pos, &(group)->devices, list, \
> +				lockdep_is_held(&(group)->mutex))

for_each_group_device() is used in bus_iommu_probe() without holding
either the mutex or the rcu lock:

static int bus_iommu_probe(const struct bus_type *bus)
{
        struct iommu_group *group, *next;
        LIST_HEAD(group_list);
        int ret;

[-- cut for short --]
	    mutex_unlock(&group->mutex);
            /*
             * FIXME: Mis-locked because the ops->probe_finalize() 
call-back
             * of some IOMMU drivers calls arm_iommu_attach_device() which
             * in-turn might call back into IOMMU core code, where it tries
             * to take group->mutex, resulting in a deadlock.
             */
             for_each_group_device(group, gdev)
                        iommu_group_do_probe_finalize(gdev->dev);
        }

        return 0;
}

Will the change above trigger a lockdep splat due to this "mis-locked"
case?"

Thanks,
baolu