[RFC PATCH 2/7] iommu: Add checked group device update helper

[Zhanpeng Zhang]

Some group-wide operations must validate every member before changing
any device. Separate iommu_group_for_each_dev() calls cannot provide
that guarantee because group membership may change between traversals.

Add iommu_group_update_devices() to keep the group membership mutex held
across a validation pass and a non-failing update pass. This provides
all-or-none validation without exposing IOMMU group internals to
callers.

Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper at bytedance.com>
---
 drivers/iommu/iommu.c | 33 +++++++++++++++++++++++++++++++++
 include/linux/iommu.h | 13 +++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index da269d10f6bf..9a6c4a7e7df6 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -1436,6 +1436,39 @@ int iommu_group_for_each_dev(struct iommu_group *group, void *data,
 }
 EXPORT_SYMBOL_GPL(iommu_group_for_each_dev);
 
+/**
+ * iommu_group_update_devices - Check and update every device in a group
+ * @group: the group
+ * @data: caller data passed to both callbacks
+ * @check: validates whether one device can be updated
+ * @update: updates one device after every check has succeeded
+ *
+ * Keep group membership stable while first checking every device and then
+ * applying an update which cannot fail. No device is updated if a check fails.
+ */
+int iommu_group_update_devices(struct iommu_group *group, void *data,
+			       int (*check)(struct device *, void *),
+			       void (*update)(struct device *, void *))
+{
+	struct group_device *device;
+	int ret = 0;
+
+	mutex_lock(&group->mutex);
+	for_each_group_device(group, device) {
+		ret = check(device->dev, data);
+		if (ret)
+			goto unlock;
+	}
+
+	for_each_group_device(group, device)
+		update(device->dev, data);
+
+unlock:
+	mutex_unlock(&group->mutex);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(iommu_group_update_devices);
+
 /**
  * iommu_group_get - Return the group for a device and increment reference
  * @dev: get the group that this device belongs to
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index e771b4a92f5b..befba0683e06 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -988,6 +988,9 @@ extern int iommu_group_add_device(struct iommu_group *group,
 extern void iommu_group_remove_device(struct device *dev);
 extern int iommu_group_for_each_dev(struct iommu_group *group, void *data,
 				    int (*fn)(struct device *, void *));
+int iommu_group_update_devices(struct iommu_group *group, void *data,
+			       int (*check)(struct device *, void *),
+			       void (*update)(struct device *, void *));
 extern struct iommu_group *iommu_group_get(struct device *dev);
 struct iommu_group *iommu_group_get_by_id(int id);
 void iommu_group_put_by_id(struct iommu_group *group);
@@ -1399,6 +1402,16 @@ static inline int iommu_group_for_each_dev(struct iommu_group *group,
 	return -ENODEV;
 }
 
+static inline int iommu_group_update_devices(struct iommu_group *group,
+					     void *data,
+					     int (*check)(struct device *,
+							  void *),
+					     void (*update)(struct device *,
+							    void *))
+{
+	return -ENODEV;
+}
+
 static inline struct iommu_group *iommu_group_get(struct device *dev)
 {
 	return NULL;
-- 
2.50.1 (Apple Git-155)