[PATCH] pmdomain: core: Wait for device link removals before dropping genpd->dev

[jaseg <git@jaseg.de>]

From: Jan Sebastian Götte <linux@jaseg.de>

genpd->dev is embedded in struct generic_pm_domain and its release
function is empty, so providers free the containing genpd with a plain
kfree() once of_genpd_remove_last() returns, ignoring its refcount.

Since genpd->dev is registered on the genpd provider bus, fw_devlink
creates device links to it, and those are torn down asynchronously.
Nothing made genpd_remove() wait for those teardowns, so the provider
could free the memory backing genpd->dev while the queued workers still
used it.

This is reachable at boot on qrb2210, where the firmware rejects PC mode
and psci_cpuidle_domain_probe() removes all the CPU PM domains before
returning -EPROBE_DEFER. The bug is asymptomatic on defconfig, but shows
up when enabling KASAN or INIT_ON_FREE_DEFAULT_ON. In some builds, it
causes the kernel to crash a few hundred ms into the boot.

Call device_link_wait_removal() before dropping the final reference. All
link removal work is queued from device_del(), via
device_links_driver_cleanup() and device_links_purge(), which precedes
genpd_free_data(), and flush_workqueue() waits for it to complete.

Note: This patch was LLM-assisted. I reproduced the issue and tested
this patch on hardware, and I did my best to verify it by hand. However,
I'm far from an expert in pmdomain, so YMMV.

Assisted-by: Claude:claude-5-opus
Fixes: 18a3a510ecfd ("pmdomain: core: Add the genpd->dev to the genpd provider bus")
Cc: stable@vger.kernel.org
Signed-off-by: Jan Sebastian Götte <linux@jaseg.de>
---
 drivers/pmdomain/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/pmdomain/core.c b/drivers/pmdomain/core.c
index 842c4169e290..4eeb980e5a40 100644
--- a/drivers/pmdomain/core.c
+++ b/drivers/pmdomain/core.c
@@ -2348,6 +2348,9 @@ static int genpd_alloc_data(struct generic_pm_domain *genpd)
 
 static void genpd_free_data(struct generic_pm_domain *genpd)
 {
+	/* Pending device link removals still reference genpd->dev. */
+	device_link_wait_removal();
+
 	put_device(&genpd->dev);
 	if (genpd->device_id != -ENXIO)
 		ida_free(&genpd_ida, genpd->device_id);
-- 
2.53.0