[PATCH RFC] driver: core: don't queue device links removal for dt overlays

[Nuno Sa via B4 Relay]

From: Nuno Sa <nuno.sa@xxxxxxxxxx>

For device links, releasing the supplier/consumer devices references
happens asynchronously in device_link_release_fn(). Hence, the possible
release of an of_node is also asynchronous. If these nodes were added
through overlays we have a problem because this does not respect the
devicetree overlays assumptions that when a changeset is
being removed in __of_changeset_entry_destroy(), it must hold the last
reference to that node. Due to the async nature of device links that
cannot be guaranteed.

Given the above, in case one of the link consumer/supplier is part of
an overlay node we call directly device_link_release_fn() instead of
queueing it. Yes, it might take some significant time for
device_link_release_fn() to complete because of synchronize_srcu() but
we would need to, anyways, wait for all OF references to be released if
we want to respect overlays assumptions.

Signed-off-by: Nuno Sa <nuno.sa@xxxxxxxxxx>
---
This RFC is a follow up of a previous one that I sent to the devicetree
folks [1]. It got rejected because it was not really fixing the root
cause of the issue (which I do agree). Please see the link where I
fully explain what the issue is.

I did also some git blaming and did saw that commit
80dd33cf72d1 ("drivers: base: Fix device link removal") introduced
queue_work() as we could be releasing the last device reference and hence
sleeping which is against SRCU callback requirements. However, that same
commit is now making use of synchronize_srcu() which may take
significant time (and I think that's the reason for the work item?).

However, given the dt overlays requirements, I'm not seeing any
reason to not be able to run device_link_release_fn() synchronously if we
detect an OVERLAY node is being released. I mean, even if we come up
(and I did some experiments in this regard) with some async mechanism to
release the OF nodes refcounts, we still need a synchronization point
somewhere.

Anyways, I would like to have some feedback on how acceptable would this
be or what else could I do so we can have a "clean" dt overlay removal.

I'm also cc'ing dts folks so they can give some comments on the new
device_node_overlay_removal() function. My goal is to try to detect when an
overlay is being removed (maybe we could even have an explicit flag for
it?) and only directly call device_link_release_fn() in that case.

[1]: https://lore.kernel.org/linux-devicetree/20230511151047.1779841-1-nuno.sa@xxxxxxxxxx/

Thanks!
- Nuno Sá
---
 drivers/base/core.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 67ba592afc77..8466b63b89c3 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -497,6 +497,18 @@ static struct attribute *devlink_attrs[] = {
 };
 ATTRIBUTE_GROUPS(devlink);
 
+static bool device_node_overlay_removal(struct device *dev)
+{
+	if (!dev_of_node(dev))
+		return false;
+	if (!of_node_check_flag(dev->of_node, OF_DETACHED))
+		return false;
+	if (!of_node_check_flag(dev->of_node, OF_OVERLAY))
+		return false;
+
+	return true;
+}
+
 static void device_link_release_fn(struct work_struct *work)
 {
 	struct device_link *link = container_of(work, struct device_link, rm_work);
@@ -532,8 +544,19 @@ static void devlink_dev_release(struct device *dev)
 	 * synchronization in device_link_release_fn() and if the consumer or
 	 * supplier devices get deleted when it runs, so put it into the "long"
 	 * workqueue.
+	 *
+	 * However, if any of the supplier, consumer nodes is being removed
+	 * through overlay removal, the expectation in
+	 * __of_changeset_entry_destroy() is for the node 'kref' to be 1 which
+	 * cannot be guaranteed with the async nature of
+	 * device_link_release_fn(). Hence, do it synchronously for the overlay
+	 * case.
 	 */
-	queue_work(system_long_wq, &link->rm_work);
+	if (device_node_overlay_removal(link->consumer) ||
+	    device_node_overlay_removal(link->supplier))
+		device_link_release_fn(&link->rm_work);
+	else
+		queue_work(system_long_wq, &link->rm_work);
 }
 
 static struct class devlink_class = {