driver core: Introduce device links reference counting

If device_link_add() is invoked multiple times with the same supplier
and consumer combo, it will create the link on first addition and
return a pointer to the already existing link on all subsequent
additions.

The semantics for device_link_del() are quite different, it deletes
the link unconditionally, so multiple invocations are not allowed.

In other words, this snippet ...

    struct device *dev1, *dev2;
    struct device_link *link1, *link2;

    link1 = device_link_add(dev1, dev2, 0);
    link2 = device_link_add(dev1, dev2, 0);

    device_link_del(link1);
    device_link_del(link2);

... causes the following crash:

    WARNING: CPU: 4 PID: 2686 at drivers/base/power/runtime.c:1611 pm_runtime_drop_link+0x40/0x50
    [...]
    list_del corruption, 0000000039b800a4->prev is LIST_POISON2 (00000000ecf79852)
    kernel BUG at lib/list_debug.c:50!

The issue isn't as arbitrary as it may seem:  Imagine a device link
which is added in both the supplier's and the consumer's ->probe hook.
The two drivers can't just call device_link_del() in their ->remove hook
without coordination.

Fix by counting multiple additions and dropping the device link only
when the last addition is unwound.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/core.c

@@ -196,8 +196,10 @@ struct device_link *device_link_add(struct device *consumer,
 	}
 
 	list_for_each_entry(link, &supplier->links.consumers, s_node)
-		if (link->consumer == consumer)
+		if (link->consumer == consumer) {
+			kref_get(&link->kref);
 			goto out;
+		}
 
 	link = kzalloc(sizeof(*link), GFP_KERNEL);
 	if (!link)
@@ -222,6 +224,7 @@ struct device_link *device_link_add(struct device *consumer,
 	link->consumer = consumer;
 	INIT_LIST_HEAD(&link->c_node);
 	link->flags = flags;
+	kref_init(&link->kref);
 
 	/* Determine the initial link state. */
 	if (flags & DL_FLAG_STATELESS) {
@@ -292,8 +295,10 @@ static void __device_link_free_srcu(struct rcu_head *rhead)
 	device_link_free(container_of(rhead, struct device_link, rcu_head));
 }
 
-static void __device_link_del(struct device_link *link)
+static void __device_link_del(struct kref *kref)
 {
+	struct device_link *link = container_of(kref, struct device_link, kref);
+
 	dev_info(link->consumer, "Dropping the link to %s\n",
 		 dev_name(link->supplier));
 
@@ -305,8 +310,10 @@ static void __device_link_del(struct device_link *link)
 	call_srcu(&device_links_srcu, &link->rcu_head, __device_link_free_srcu);
 }
 #else /* !CONFIG_SRCU */
-static void __device_link_del(struct device_link *link)
+static void __device_link_del(struct kref *kref)
 {
+	struct device_link *link = container_of(kref, struct device_link, kref);
+
 	dev_info(link->consumer, "Dropping the link to %s\n",
 		 dev_name(link->supplier));
 
@@ -324,13 +331,15 @@ static void __device_link_del(struct device_link *link)
  * @link: Device link to delete.
  *
  * The caller must ensure proper synchronization of this function with runtime
- * PM.
+ * PM.  If the link was added multiple times, it needs to be deleted as often.
+ * Care is required for hotplugged devices:  Their links are purged on removal
+ * and calling device_link_del() is then no longer allowed.
  */
 void device_link_del(struct device_link *link)
 {
 	device_links_write_lock();
 	device_pm_lock();
-	__device_link_del(link);
+	kref_put(&link->kref, __device_link_del);
 	device_pm_unlock();
 	device_links_write_unlock();
 }
@@ -444,7 +453,7 @@ static void __device_links_no_driver(struct device *dev)
 			continue;
 
 		if (link->flags & DL_FLAG_AUTOREMOVE)
-			__device_link_del(link);
+			kref_put(&link->kref, __device_link_del);
 		else if (link->status != DL_STATE_SUPPLIER_UNBIND)
 			WRITE_ONCE(link->status, DL_STATE_AVAILABLE);
 	}
@@ -597,13 +606,13 @@ static void device_links_purge(struct device *dev)
 
 	list_for_each_entry_safe_reverse(link, ln, &dev->links.suppliers, c_node) {
 		WARN_ON(link->status == DL_STATE_ACTIVE);
-		__device_link_del(link);
+		__device_link_del(&link->kref);
 	}
 
 	list_for_each_entry_safe_reverse(link, ln, &dev->links.consumers, s_node) {
 		WARN_ON(link->status != DL_STATE_DORMANT &&
 			link->status != DL_STATE_NONE);
-		__device_link_del(link);
+		__device_link_del(&link->kref);
 	}
 
 	device_links_write_unlock();

include/linux/device.h

@@ -769,6 +769,7 @@ enum device_link_state {
  * @status: The state of the link (with respect to the presence of drivers).
  * @flags: Link flags.
  * @rpm_active: Whether or not the consumer device is runtime-PM-active.
+ * @kref: Count repeated addition of the same link.
  * @rcu_head: An RCU head to use for deferred execution of SRCU callbacks.
  */
 struct device_link {
@@ -779,6 +780,7 @@ struct device_link {
 	enum device_link_state status;
 	u32 flags;
 	bool rpm_active;
+	struct kref kref;
 #ifdef CONFIG_SRCU
 	struct rcu_head rcu_head;
 #endif