[RFC PATCH v2 1/2] fpga: add a module owner field to fpga_manager and fpga_manager_ops

[Marco Pagani <marpagan@redhat.com>]

Add a module *owner field to the fpga_manager_ops and fpga_manager
structs to protect the fpga manager against the unloading of the
low-level control module while someone is holding a reference to the
manager device. Low-level control modules should statically set the
owner field of the fpga_manager_ops struct to THIS_MODULE. Then, when
the manager is registered using fpga_mgr_register(), the value is copied
into the owner field of the fpga_manager struct (that contains the
device context). In this way, the manager can later use it in
fpga_mgr_get() to take the low-level module's refcount. To prevent races
while unloading the low-level control module, fpga_mgr_get() and part of
the fpga_mgr_unregister() methods are protected with a mutex.

Other changes: move put_device() from __fpga_mgr_get() to fpga_mgr_get()
and of_fpga_mgr_get() to improve code clarity.

Fixes: 654ba4cc0f3e ("fpga manager: ensure lifetime with of_fpga_mgr_get")
Signed-off-by: Marco Pagani <marpagan@redhat.com>
---
 drivers/fpga/fpga-mgr.c       | 56 +++++++++++++++++++++++++----------
 include/linux/fpga/fpga-mgr.h |  4 +++
 2 files changed, 44 insertions(+), 16 deletions(-)

diff --git a/drivers/fpga/fpga-mgr.c b/drivers/fpga/fpga-mgr.c
index 06651389c592..608605d59860 100644
--- a/drivers/fpga/fpga-mgr.c
+++ b/drivers/fpga/fpga-mgr.c
@@ -21,6 +21,8 @@
 static DEFINE_IDA(fpga_mgr_ida);
 static const struct class fpga_mgr_class;
 
+static DEFINE_MUTEX(mgr_lock);
+
 struct fpga_mgr_devres {
 	struct fpga_manager *mgr;
 };
@@ -667,17 +669,15 @@ ATTRIBUTE_GROUPS(fpga_mgr);
 static struct fpga_manager *__fpga_mgr_get(struct device *dev)
 {
 	struct fpga_manager *mgr;
+	struct module *owner;
 
 	mgr = to_fpga_manager(dev);
+	owner = mgr->owner;
 
-	if (!try_module_get(dev->parent->driver->owner))
-		goto err_dev;
+	if (owner && !try_module_get(owner))
+		mgr = ERR_PTR(-ENODEV);
 
 	return mgr;
-
-err_dev:
-	put_device(dev);
-	return ERR_PTR(-ENODEV);
 }
 
 static int fpga_mgr_dev_match(struct device *dev, const void *data)
@@ -693,12 +693,22 @@ static int fpga_mgr_dev_match(struct device *dev, const void *data)
  */
 struct fpga_manager *fpga_mgr_get(struct device *dev)
 {
-	struct device *mgr_dev = class_find_device(&fpga_mgr_class, NULL, dev,
-						   fpga_mgr_dev_match);
+	struct fpga_manager *mgr = ERR_PTR(-ENODEV);
+	struct device *mgr_dev;
+
+	mutex_lock(&mgr_lock);
+
+	mgr_dev = class_find_device(&fpga_mgr_class, NULL, dev, fpga_mgr_dev_match);
 	if (!mgr_dev)
-		return ERR_PTR(-ENODEV);
+		goto out;
+
+	mgr = __fpga_mgr_get(mgr_dev);
+	if (IS_ERR(mgr))
+		put_device(mgr_dev);
 
-	return __fpga_mgr_get(mgr_dev);
+out:
+	mutex_unlock(&mgr_lock);
+	return mgr;
 }
 EXPORT_SYMBOL_GPL(fpga_mgr_get);
 
@@ -711,13 +721,22 @@ EXPORT_SYMBOL_GPL(fpga_mgr_get);
  */
 struct fpga_manager *of_fpga_mgr_get(struct device_node *node)
 {
-	struct device *dev;
+	struct fpga_manager *mgr = ERR_PTR(-ENODEV);
+	struct device *mgr_dev;
+
+	mutex_lock(&mgr_lock);
+
+	mgr_dev = class_find_device_by_of_node(&fpga_mgr_class, node);
+	if (!mgr_dev)
+		goto out;
 
-	dev = class_find_device_by_of_node(&fpga_mgr_class, node);
-	if (!dev)
-		return ERR_PTR(-ENODEV);
+	mgr = __fpga_mgr_get(mgr_dev);
+	if (IS_ERR(mgr))
+		put_device(mgr_dev);
 
-	return __fpga_mgr_get(dev);
+out:
+	mutex_unlock(&mgr_lock);
+	return mgr;
 }
 EXPORT_SYMBOL_GPL(of_fpga_mgr_get);
 
@@ -727,7 +746,7 @@ EXPORT_SYMBOL_GPL(of_fpga_mgr_get);
  */
 void fpga_mgr_put(struct fpga_manager *mgr)
 {
-	module_put(mgr->dev.parent->driver->owner);
+	module_put(mgr->owner);
 	put_device(&mgr->dev);
 }
 EXPORT_SYMBOL_GPL(fpga_mgr_put);
@@ -806,6 +825,7 @@ fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *in
 
 	mgr->name = info->name;
 	mgr->mops = info->mops;
+	mgr->owner = info->mops->owner;
 	mgr->priv = info->priv;
 	mgr->compat_id = info->compat_id;
 
@@ -888,7 +908,11 @@ void fpga_mgr_unregister(struct fpga_manager *mgr)
 	 */
 	fpga_mgr_fpga_remove(mgr);
 
+	mutex_lock(&mgr_lock);
+
 	device_unregister(&mgr->dev);
+
+	mutex_unlock(&mgr_lock);
 }
 EXPORT_SYMBOL_GPL(fpga_mgr_unregister);
 
diff --git a/include/linux/fpga/fpga-mgr.h b/include/linux/fpga/fpga-mgr.h
index 54f63459efd6..eaf6e072dbc0 100644
--- a/include/linux/fpga/fpga-mgr.h
+++ b/include/linux/fpga/fpga-mgr.h
@@ -162,6 +162,7 @@ struct fpga_manager_info {
  * @write_complete: set FPGA to operating state after writing is done
  * @fpga_remove: optional: Set FPGA into a specific state during driver remove
  * @groups: optional attribute groups.
+ * @owner: owner module.
  *
  * fpga_manager_ops are the low level functions implemented by a specific
  * fpga manager driver.  The optional ones are tested for NULL before being
@@ -184,6 +185,7 @@ struct fpga_manager_ops {
 			      struct fpga_image_info *info);
 	void (*fpga_remove)(struct fpga_manager *mgr);
 	const struct attribute_group **groups;
+	struct module *owner;
 };
 
 /* FPGA manager status: Partial/Full Reconfiguration errors */
@@ -201,6 +203,7 @@ struct fpga_manager_ops {
  * @state: state of fpga manager
  * @compat_id: FPGA manager id for compatibility check.
  * @mops: pointer to struct of fpga manager ops
+ * @owner: owner module.
  * @priv: low level driver private date
  */
 struct fpga_manager {
@@ -210,6 +213,7 @@ struct fpga_manager {
 	enum fpga_mgr_states state;
 	struct fpga_compat_id *compat_id;
 	const struct fpga_manager_ops *mops;
+	struct module *owner;
 	void *priv;
 };
 
-- 
2.42.0