[PATCH v4 2/2] workqueue: Add warnings and ensure one among WQ_PERCPU or WQ_UNBOUND is present

[Marco Crivellari]

Currently there are no checks in order to enforce the use of one between
WQ_PERCPU or WQ_UNBOUND.

So act as following:
- if neither of them is present, set WQ_PERCPU
- if both are present, remove WQ_PERCPU

Along with this change, WARN_ONCE(), so that the code still uses both or
neither of them, can be changed.

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 | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 0b55f8009eed..d4799e93d10f 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5826,7 +5826,7 @@ static struct workqueue_struct *__alloc_workqueue(const char *fmt,
 
 	/* see the comment above the definition of WQ_POWER_EFFICIENT */
 	if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient)
-		flags |= WQ_UNBOUND;
+		flags = (flags & ~WQ_PERCPU) | WQ_UNBOUND;
 
 	/* allocate wq and format name */
 	if (flags & WQ_UNBOUND)
@@ -5850,6 +5850,23 @@ static struct workqueue_struct *__alloc_workqueue(const char *fmt,
 		pr_warn_once("workqueue: name exceeds WQ_NAME_LEN. Truncating to: %s\n",
 			     wq->name);
 
+	/*
+	 * One among WQ_PERCPU and WQ_UNBOUND must be set, but not both.
+	 * - If neither is set, default to WQ_PERCPU
+	 * - If both are set, default to WQ_UNBOUND
+	 *
+	 * This code can be removed after workqueue are unbound by default
+	 */
+	if (unlikely(!(flags & (WQ_UNBOUND | WQ_PERCPU)))) {
+		WARN_ONCE(1, "workqueue: %s is using neither WQ_PERCPU or WQ_UNBOUND. "
+			  "Setting WQ_PERCPU.\n", wq->name);
+		flags |= WQ_PERCPU;
+	} else if(unlikely((flags & WQ_PERCPU) && (flags & WQ_UNBOUND))) {
+		WARN_ONCE(1, "workqueue: %s uses both WQ_PERCPU and WQ_UNBOUND. "
+			  "Dropped WQ_PERCPU, keeping WQ_UNBOUND.\n", wq->name);
+		flags &= (~WQ_PERCPU);
+	}
+
 	if (flags & WQ_BH) {
 		/*
 		 * BH workqueues always share a single execution context per CPU
-- 
2.54.0