[RFC PATCH v2 1/2] workqueue: Add warnings and fallback if system_{unbound}_wq is used

[Marco Crivellari]

Currently many users transitioned already to the new introduced workqueue
(system_percpu_wq, system_dfl_wq), but there are new users who still use the
older system_wq and system_unbound_wq.

This change try to push this transition forward, by warning whether the old
workqueus are used and redirecting them old used workqueue with the appropriate
new one.

Link: https://lore.kernel.org/all/20250221112003.1dSuoGyc@xxxxxxxxxxxxx/
Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Marco Crivellari <marco.crivellari@xxxxxxxx>
---
 kernel/workqueue.c | 72 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 33b721a9af02..e17617e205b1 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2437,6 +2437,24 @@ bool queue_work_on(int cpu, struct workqueue_struct *wq,
 	bool ret = false;
 	unsigned long irq_flags;
 
+	/*
+	 * NOTE: These checks are here to assure that no users will still
+	 * rely on system_wq and system_unbound_wq.
+	 * They can be removed along with those workqueue when the
+	 * time comes.
+	 */
+	if (unlikely(wq == system_wq)) {
+		pr_warn_once("workqueue: system_wq will be removed shortly. "
+			"Use system_percpu_wq instead. Caller: %ps\n",
+			__builtin_return_address(0));
+		wq = system_percpu_wq;
+	} else if (unlikely(wq == system_unbound_wq)) {
+		pr_warn_once("workqueue: system_unbound_wq will be removed shortly. "
+			"Use system_dfl_wq instead. Caller: %ps\n",
+			__builtin_return_address(0));
+		wq = system_dfl_wq;
+	}
+
 	local_irq_save(irq_flags);
 
 	if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)) &&
@@ -2604,6 +2622,24 @@ bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
 	bool ret = false;
 	unsigned long irq_flags;
 
+	/*
+	 * NOTE: These checks are here to assure that no users will still
+	 * rely on system_wq and system_unbound_wq.
+	 * They can be removed along with those workqueue when the
+	 * time comes.
+	 */
+	if (unlikely(wq == system_wq)) {
+		pr_warn_once("workqueue: system_wq will be removed shortly. "
+			"Use system_percpu_wq instead. Caller: %ps\n",
+			__builtin_return_address(0));
+		wq = system_percpu_wq;
+	} else if (unlikely(wq == system_unbound_wq)) {
+		pr_warn_once("workqueue: system_unbound_wq will be removed shortly. "
+			"Use system_dfl_wq instead. Caller: %ps\n",
+			__builtin_return_address(0));
+		wq = system_dfl_wq;
+	}
+
 	/* read the comment in __queue_work() */
 	local_irq_save(irq_flags);
 
@@ -2642,6 +2678,24 @@ bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
 	unsigned long irq_flags;
 	bool ret;
 
+	/*
+	 * NOTE: These checks are here to assure that no users will still
+	 * rely on system_wq and system_unbound_wq.
+	 * They can be removed along with those workqueue when the
+	 * time comes.
+	 */
+	if (unlikely(wq == system_wq)) {
+		pr_warn_once("workqueue: system_wq will be removed shortly. "
+		      "Use system_percpu_wq instead. Caller: %ps\n",
+		       __builtin_return_address(0));
+		wq = system_percpu_wq;
+	} else if (unlikely(wq == system_unbound_wq)) {
+		pr_warn_once("workqueue: system_unbound_wq will be removed shortly. "
+		      "Use system_dfl_wq instead. Caller: %ps\n",
+		       __builtin_return_address(0));
+		wq = system_dfl_wq;
+	}
+
 	ret = work_grab_pending(&dwork->work, WORK_CANCEL_DELAYED, &irq_flags);
 
 	if (!clear_pending_if_disabled(&dwork->work))
@@ -2676,6 +2730,24 @@ bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork)
 {
 	struct work_struct *work = &rwork->work;
 
+	/*
+	 * NOTE: These checks are here to assure that no users will still
+	 * rely on system_wq and system_unbound_wq.
+	 * They can be removed along with those workqueue when the
+	 * time comes.
+	 */
+	if (unlikely(wq == system_wq)) {
+		pr_warn_once("workqueue: system_wq will be removed shortly. "
+			"Use system_percpu_wq instead. Caller: %ps\n",
+			__builtin_return_address(0));
+		wq = system_percpu_wq;
+	} else if (unlikely(wq == system_unbound_wq)) {
+		pr_warn_once("workqueue: system_unbound_wq will be removed shortly. "
+			"Use system_dfl_wq instead. Caller: %ps\n",
+			__builtin_return_address(0));
+		wq = system_dfl_wq;
+	}
+
 	/*
 	 * rcu_work can't be canceled or disabled. Warn if the user reached
 	 * inside @rwork and disabled the inner work.
-- 
2.54.0