[RFC] driver: core: don't queue device links removal for dt overlays
Commit Message
From: Nuno Sa <nuno.sa@analog.com>
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@analog.com>
---
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@analog.com/
Thanks!
- Nuno Sá
---
drivers/base/core.c | 25 ++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)
@@ -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 = {