Re: [PATCH 06/10] iommu: Replace iommu_device_lock with iommu_probe_device_lock

[Robin Murphy <robin.murphy@arm.com>]

On 29/11/2023 12:48 am, Jason Gunthorpe wrote:
> The iommu_device_lock protects the iommu_device_list which is only read by
> iommu_ops_from_fwnode().
> 
> This is now always called under the iommu_probe_device_lock, so we don't
> need to double lock the linked list. Use the iommu_probe_device_lock on
> the write side too.

Please no, iommu_probe_device_lock() is a hack and we need to remove the 
*reason* it exists at all. And IMO just because iommu_present() is 
deprecated doesn't justify making it look utterly nonsensical - in no 
way does that have any relationship with probe_device, much less need to 
serialise against it!

Thanks,
Robin.

> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   drivers/iommu/iommu.c | 30 +++++++++++++-----------------
>   1 file changed, 13 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 08f29a1dfcd5f8..9557c2ec08d915 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -146,7 +146,6 @@ struct iommu_group_attribute iommu_group_attr_##_name =		\
>   	container_of(_kobj, struct iommu_group, kobj)
>   
>   static LIST_HEAD(iommu_device_list);
> -static DEFINE_SPINLOCK(iommu_device_lock);
>   
>   static const struct bus_type * const iommu_buses[] = {
>   	&platform_bus_type,
> @@ -262,9 +261,9 @@ int iommu_device_register(struct iommu_device *iommu,
>   	if (hwdev)
>   		iommu->fwnode = dev_fwnode(hwdev);
>   
> -	spin_lock(&iommu_device_lock);
> +	mutex_lock(&iommu_probe_device_lock);
>   	list_add_tail(&iommu->list, &iommu_device_list);
> -	spin_unlock(&iommu_device_lock);
> +	mutex_unlock(&iommu_probe_device_lock);
>   
>   	for (int i = 0; i < ARRAY_SIZE(iommu_buses) && !err; i++)
>   		err = bus_iommu_probe(iommu_buses[i]);
> @@ -279,9 +278,9 @@ void iommu_device_unregister(struct iommu_device *iommu)
>   	for (int i = 0; i < ARRAY_SIZE(iommu_buses); i++)
>   		bus_for_each_dev(iommu_buses[i], NULL, iommu, remove_iommu_group);
>   
> -	spin_lock(&iommu_device_lock);
> +	mutex_lock(&iommu_probe_device_lock);
>   	list_del(&iommu->list);
> -	spin_unlock(&iommu_device_lock);
> +	mutex_unlock(&iommu_probe_device_lock);
>   
>   	/* Pairs with the alloc in generic_single_device_group() */
>   	iommu_group_put(iommu->singleton_group);
> @@ -316,9 +315,9 @@ int iommu_device_register_bus(struct iommu_device *iommu,
>   	if (err)
>   		return err;
>   
> -	spin_lock(&iommu_device_lock);
> +	mutex_lock(&iommu_probe_device_lock);
>   	list_add_tail(&iommu->list, &iommu_device_list);
> -	spin_unlock(&iommu_device_lock);
> +	mutex_unlock(&iommu_probe_device_lock);
>   
>   	err = bus_iommu_probe(bus);
>   	if (err) {
> @@ -2033,9 +2032,9 @@ bool iommu_present(const struct bus_type *bus)
>   
>   	for (int i = 0; i < ARRAY_SIZE(iommu_buses); i++) {
>   		if (iommu_buses[i] == bus) {
> -			spin_lock(&iommu_device_lock);
> +			mutex_lock(&iommu_probe_device_lock);
>   			ret = !list_empty(&iommu_device_list);
> -			spin_unlock(&iommu_device_lock);
> +			mutex_unlock(&iommu_probe_device_lock);
>   		}
>   	}
>   	return ret;
> @@ -2980,17 +2979,14 @@ EXPORT_SYMBOL_GPL(iommu_default_passthrough);
>   
>   const struct iommu_ops *iommu_ops_from_fwnode(struct fwnode_handle *fwnode)
>   {
> -	const struct iommu_ops *ops = NULL;
>   	struct iommu_device *iommu;
>   
> -	spin_lock(&iommu_device_lock);
> +	lockdep_assert_held(&iommu_probe_device_lock);
> +
>   	list_for_each_entry(iommu, &iommu_device_list, list)
> -		if (iommu->fwnode == fwnode) {
> -			ops = iommu->ops;
> -			break;
> -		}
> -	spin_unlock(&iommu_device_lock);
> -	return ops;
> +		if (iommu->fwnode == fwnode)
> +			return iommu->ops;
> +	return NULL;
>   }
>   
>   int iommu_fwspec_init(struct device *dev, struct fwnode_handle *iommu_fwnode,