[PATCH v3] driver core: Fix SYNC_STATE_ONLY device link implementation

[Saravana Kannan <saravanak@google.com>]

When SYNC_STATE_ONLY support was added in commit 05ef983e0d65 ("driver
core: Add device link support for SYNC_STATE_ONLY flag"),
device_link_add() incorrectly skipped adding the new SYNC_STATE_ONLY
device link to the supplier's and consumer's "device link" list.

This causes multiple issues:
- The device link is lost forever from driver core if the caller
  didn't keep track of it (caller typically isn't expected to). This is
  a memory leak.
- The device link is also never visible to any other code path after
  device_link_add() returns.

If we fix the "device link" list handling, that exposes a bunch of
issues.

1. The device link "status" state management code rightfully doesn't
handle the case where a DL_FLAG_MANAGED device link exists between a
supplier and consumer, but the consumer manages to probe successfully
before the supplier. The addition of DL_FLAG_SYNC_STATE_ONLY links break
this assumption. This causes device_links_driver_bound() to throw a
warning when this happens.

Since DL_FLAG_SYNC_STATE_ONLY device links are mainly used for creating
proxy device links for child device dependencies and aren't useful once
the consumer device probes successfully, this patch just deletes
DL_FLAG_SYNC_STATE_ONLY device links once its consumer device probes.
This way, we avoid the warning, free up some memory and avoid
complicating the device links "status" state management code.

2. Creating a DL_FLAG_STATELESS device link between two devices that
already have a DL_FLAG_SYNC_STATE_ONLY device link will result in the
DL_FLAG_STATELESS flag not getting set correctly. This patch also fixes
this.

Lastly, this patch also fixes minor whitespace issues.

Cc: stable@vger.kernel.org
Fixes: 05ef983e0d65 ("driver core: Add device link support for SYNC_STATE_ONLY flag")
Signed-off-by: Saravana Kannan <saravanak@google.com>
---
v3:
- Added this changelog text
v2:
- Delete DL_FLAG_SYNC_STATE_ONLY device links on consumer probe
- Set DL_FLAG_STATELESS correct when added to an existing
  DL_FLAG_SYNC_STATE_ONLY device link.
v1:
- Add device link to list
- Minor whitespace fixes

 drivers/base/core.c | 61 +++++++++++++++++++++++++++++----------------
 1 file changed, 39 insertions(+), 22 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 84c569726d75..f804e561e0a2 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -363,13 +363,12 @@ struct device_link *device_link_add(struct device *consumer,
 
 		if (flags & DL_FLAG_STATELESS) {
 			kref_get(&link->kref);
+			link->flags |= DL_FLAG_STATELESS;
 			if (link->flags & DL_FLAG_SYNC_STATE_ONLY &&
-			    !(link->flags & DL_FLAG_STATELESS)) {
-				link->flags |= DL_FLAG_STATELESS;
+			    !(link->flags & DL_FLAG_STATELESS))
 				goto reorder;
-			} else {
+			else
 				goto out;
-			}
 		}
 
 		/*
@@ -436,12 +435,16 @@ struct device_link *device_link_add(struct device *consumer,
 	    flags & DL_FLAG_PM_RUNTIME)
 		pm_runtime_resume(supplier);
 
+	list_add_tail_rcu(&link->s_node, &supplier->links.consumers);
+	list_add_tail_rcu(&link->c_node, &consumer->links.suppliers);
+
 	if (flags & DL_FLAG_SYNC_STATE_ONLY) {
 		dev_dbg(consumer,
 			"Linked as a sync state only consumer to %s\n",
 			dev_name(supplier));
 		goto out;
 	}
+
 reorder:
 	/*
 	 * Move the consumer and all of the devices depending on it to the end
@@ -452,12 +455,9 @@ struct device_link *device_link_add(struct device *consumer,
 	 */
 	device_reorder_to_tail(consumer, NULL);
 
-	list_add_tail_rcu(&link->s_node, &supplier->links.consumers);
-	list_add_tail_rcu(&link->c_node, &consumer->links.suppliers);
-
 	dev_dbg(consumer, "Linked as a consumer to %s\n", dev_name(supplier));
 
- out:
+out:
 	device_pm_unlock();
 	device_links_write_unlock();
 
@@ -832,6 +832,13 @@ static void __device_links_supplier_defer_sync(struct device *sup)
 		list_add_tail(&sup->links.defer_sync, &deferred_sync);
 }
 
+static void device_link_drop_managed(struct device_link *link)
+{
+	link->flags &= ~DL_FLAG_MANAGED;
+	WRITE_ONCE(link->status, DL_STATE_NONE);
+	kref_put(&link->kref, __device_link_del);
+}
+
 /**
  * device_links_driver_bound - Update device links after probing its driver.
  * @dev: Device to update the links for.
@@ -845,7 +852,7 @@ static void __device_links_supplier_defer_sync(struct device *sup)
  */
 void device_links_driver_bound(struct device *dev)
 {
-	struct device_link *link;
+	struct device_link *link, *ln;
 	LIST_HEAD(sync_list);
 
 	/*
@@ -885,18 +892,35 @@ void device_links_driver_bound(struct device *dev)
 	else
 		__device_links_queue_sync_state(dev, &sync_list);
 
-	list_for_each_entry(link, &dev->links.suppliers, c_node) {
+	list_for_each_entry_safe(link, ln, &dev->links.suppliers, c_node) {
+		struct device *supplier;
+
 		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
-		WARN_ON(link->status != DL_STATE_CONSUMER_PROBE);
-		WRITE_ONCE(link->status, DL_STATE_ACTIVE);
+		supplier = link->supplier;
+		if (link->flags & DL_FLAG_SYNC_STATE_ONLY) {
+			/*
+			 * When DL_FLAG_SYNC_STATE_ONLY is set, it means no
+			 * other DL_MANAGED_LINK_FLAGS have been set. So, it's
+			 * save to drop the managed link completely.
+			 */
+			device_link_drop_managed(link);
+		} else {
+			WARN_ON(link->status != DL_STATE_CONSUMER_PROBE);
+			WRITE_ONCE(link->status, DL_STATE_ACTIVE);
+		}
 
+		/*
+		 * This needs to be done even for the deleted
+		 * DL_FLAG_SYNC_STATE_ONLY device link in case it was the last
+		 * device link that was preventing the supplier from getting a
+		 * sync_state() call.
+		 */
 		if (defer_sync_state_count)
-			__device_links_supplier_defer_sync(link->supplier);
+			__device_links_supplier_defer_sync(supplier);
 		else
-			__device_links_queue_sync_state(link->supplier,
-							&sync_list);
+			__device_links_queue_sync_state(supplier, &sync_list);
 	}
 
 	dev->links.status = DL_DEV_DRIVER_BOUND;
@@ -906,13 +930,6 @@ void device_links_driver_bound(struct device *dev)
 	device_links_flush_sync_list(&sync_list, dev);
 }
 
-static void device_link_drop_managed(struct device_link *link)
-{
-	link->flags &= ~DL_FLAG_MANAGED;
-	WRITE_ONCE(link->status, DL_STATE_NONE);
-	kref_put(&link->kref, __device_link_del);
-}
-
 /**
  * __device_links_no_driver - Update links of a device without a driver.
  * @dev: Device without a drvier.
-- 
2.26.2.761.g0e0b3e54be-goog