[RFC PATCH v3 1/2] fpga: add an owner field and use it to take the low-level module's refcount

[Marco Pagani <marpagan@redhat.com>]

Add a module owner field to the fpga_manager_ops struct and use it to
take the low-level control module's refcount instead of relying on the
parent device's driver pointer. Low-level control modules should
statically set the owner field to THIS_MODULE. To detect when the owner
module pointer becomes stale, set the mops pointer to null during
fpga_mgr_unregister() (called by the low-level module exit function) and
test it before taking the module's refcount. Use a mutex to avoid a
crash that can happen if __fpga_mgr_get() gets suspended between testing
the mops pointer and taking the low-level refcount and then resumes when
the low-level module has already been freed.

Thanks to Xu Yilun for suggesting the locking pattern.

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       | 50 ++++++++++++++++++++++++-----------
 include/linux/fpga/fpga-mgr.h |  4 +++
 2 files changed, 38 insertions(+), 16 deletions(-)

diff --git a/drivers/fpga/fpga-mgr.c b/drivers/fpga/fpga-mgr.c
index 06651389c592..a32b7d40080d 100644
--- a/drivers/fpga/fpga-mgr.c
+++ b/drivers/fpga/fpga-mgr.c
@@ -664,20 +664,20 @@ static struct attribute *fpga_mgr_attrs[] = {
 };
 ATTRIBUTE_GROUPS(fpga_mgr);
 
-static struct fpga_manager *__fpga_mgr_get(struct device *dev)
+static struct fpga_manager *__fpga_mgr_get(struct device *mgr_dev)
 {
 	struct fpga_manager *mgr;
 
-	mgr = to_fpga_manager(dev);
+	mgr = to_fpga_manager(mgr_dev);
 
-	if (!try_module_get(dev->parent->driver->owner))
-		goto err_dev;
+	mutex_lock(&mgr->mops_mutex);
 
-	return mgr;
+	if (!mgr->mops || !try_module_get(mgr->mops->owner))
+		mgr = ERR_PTR(-ENODEV);
 
-err_dev:
-	put_device(dev);
-	return ERR_PTR(-ENODEV);
+	mutex_unlock(&mgr->mops_mutex);
+
+	return mgr;
 }
 
 static int fpga_mgr_dev_match(struct device *dev, const void *data)
@@ -693,12 +693,18 @@ 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;
+	struct device *mgr_dev;
+
+	mgr_dev = class_find_device(&fpga_mgr_class, NULL, dev, fpga_mgr_dev_match);
 	if (!mgr_dev)
 		return ERR_PTR(-ENODEV);
 
-	return __fpga_mgr_get(mgr_dev);
+	mgr = __fpga_mgr_get(mgr_dev);
+	if (IS_ERR(mgr))
+		put_device(mgr_dev);
+
+	return mgr;
 }
 EXPORT_SYMBOL_GPL(fpga_mgr_get);
 
@@ -711,13 +717,18 @@ EXPORT_SYMBOL_GPL(fpga_mgr_get);
  */
 struct fpga_manager *of_fpga_mgr_get(struct device_node *node)
 {
-	struct device *dev;
+	struct fpga_manager *mgr;
+	struct device *mgr_dev;
 
-	dev = class_find_device_by_of_node(&fpga_mgr_class, node);
-	if (!dev)
+	mgr_dev = class_find_device_by_of_node(&fpga_mgr_class, node);
+	if (!mgr_dev)
 		return ERR_PTR(-ENODEV);
 
-	return __fpga_mgr_get(dev);
+	mgr = __fpga_mgr_get(mgr_dev);
+	if (IS_ERR(mgr))
+		put_device(mgr_dev);
+
+	return mgr;
 }
 EXPORT_SYMBOL_GPL(of_fpga_mgr_get);
 
@@ -727,7 +738,7 @@ 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->mops->owner);
 	put_device(&mgr->dev);
 }
 EXPORT_SYMBOL_GPL(fpga_mgr_put);
@@ -803,6 +814,7 @@ fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *in
 	}
 
 	mutex_init(&mgr->ref_mutex);
+	mutex_init(&mgr->mops_mutex);
 
 	mgr->name = info->name;
 	mgr->mops = info->mops;
@@ -888,6 +900,12 @@ void fpga_mgr_unregister(struct fpga_manager *mgr)
 	 */
 	fpga_mgr_fpga_remove(mgr);
 
+	mutex_lock(&mgr->mops_mutex);
+
+	mgr->mops = NULL;
+
+	mutex_unlock(&mgr->mops_mutex);
+
 	device_unregister(&mgr->dev);
 }
 EXPORT_SYMBOL_GPL(fpga_mgr_unregister);
diff --git a/include/linux/fpga/fpga-mgr.h b/include/linux/fpga/fpga-mgr.h
index 54f63459efd6..b4d9413cb444 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 containing the ops.
  *
  * 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
+ * @mops_mutex: protects mops from low-level module removal
  * @priv: low level driver private date
  */
 struct fpga_manager {
@@ -209,6 +212,7 @@ struct fpga_manager {
 	struct mutex ref_mutex;
 	enum fpga_mgr_states state;
 	struct fpga_compat_id *compat_id;
+	struct mutex mops_mutex;
 	const struct fpga_manager_ops *mops;
 	void *priv;
 };
-- 
2.43.0