Messages in this thread Patch in this message | | | From | Nuno Sa via B4 Relay <> | Date | Mon, 27 Nov 2023 16:23:13 +0100 | Subject | [PATCH RFC] driver: core: don't queue device links removal for dt overlays |
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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(-)
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 = {
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