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

[Robin Murphy]

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 <j...@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,