[PATCH v2 02/58] driver core: retain the owner of device release callbacks

[James Hilliard <james.hilliard1@gmail.com>]

Device references can outlive the driver or subsystem operation that
unregistered the device. A modular release callback needs its code to
remain loaded until it returns. Dropping its last module reference inside
the callback permits concurrent unload before the function epilogue has
finished.

Add an optional release_owner to struct device. device_initialize() takes
the reference before the device can be published. The built-in device
core caches the owner before invoking the release callback and drops the
reference after the callback returns, allowing the callback to free the
device itself. This also covers initialized devices that were never added
and device_add() failures.

MTD will use this to keep its release callback available after retained
device references outlive driver unbind.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: driver-core@lists.linux.dev
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/base/core.c    | 9 +++++++++
 include/linux/device.h | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 4c0c373998a1..03f8bf69219a 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -2617,6 +2617,7 @@ static void device_release(struct kobject *kobj)
 {
 	struct device *dev = kobj_to_dev(kobj);
 	struct device_private *p = dev->p;
+	struct module *owner = dev->release_owner;
 
 	/*
 	 * Some platform devices are driven without driver attached
@@ -2642,6 +2643,7 @@ static void device_release(struct kobject *kobj)
 		WARN(1, KERN_ERR "Device '%s' does not have a release() function, it is broken and must be fixed. See Documentation/core-api/kobject.rst.\n",
 			dev_name(dev));
 	kfree(p);
+	module_put(owner);
 }
 
 static const struct ns_common *device_namespace(const struct kobject *kobj)
@@ -3225,11 +3227,18 @@ static void klist_children_put(struct klist_node *n)
  * approach is to use kzalloc() to allocate the structure containing
  * @dev.
  *
+ * If @dev->release_owner is set, the caller must keep that module loaded
+ * until this function acquires a reference. The core drops the reference
+ * after the release callback returns, including when device_add() fails.
+ * A module whose initialization fails must still drain its devices before
+ * returning the error, since module references do not prevent that unload.
+ *
  * NOTE: Use put_device() to give up your reference instead of freeing
  * @dev directly once you have called this function.
  */
 void device_initialize(struct device *dev)
 {
+	__module_get(dev->release_owner);
 	dev->kobj.kset = devices_kset;
 	kobject_init(&dev->kobj, &device_ktype);
 	INIT_LIST_HEAD(&dev->dma_pools);
diff --git a/include/linux/device.h b/include/linux/device.h
index aee79fd6b32b..cd39ebf47ee7 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -683,6 +683,9 @@ enum struct_device_flags {
  * @release:	Callback to free the device after all references have
  * 		gone away. This should be set by the allocator of the
  * 		device (i.e. the bus driver that discovered the device).
+ * @release_owner: Optional module containing the release callback. Set before
+ *		device_initialize() and leave unchanged for the device's lifetime.
+ *		The core holds a reference until the release callback returns.
  * @iommu_group: IOMMU group the device belongs to.
  * @iommu:	Per device generic IOMMU runtime data
  * @physical_location: Describes physical location of the device connection
@@ -787,6 +790,7 @@ struct device {
 	const struct attribute_group **groups;	/* optional groups */
 
 	void	(*release)(struct device *dev);
+	struct module		*release_owner;
 	struct iommu_group	*iommu_group;
 	struct dev_iommu	*iommu;
 

-- 
2.53.0