[PATCH 1/2] workqueue: Allow to expose ordered workqueues via sysfs

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Ordered workqueues are not exposed via sysfs because the 'max_active'
attribute changes the number actives worker. More than one active worker
can break ordering guarantees.

This can be avoided by forbidding writes the file for ordered
workqueues. Exposing it via sysfs allows to alter other attributes such
as the cpumask on which CPU the worker can run.

The 'max_active' value shouldn't be changed for BH worker because the
core never spawns additional worker and the worker itself can not be
preempted. So this make no sense.

Allow to expose ordered workqueues via sysfs if requested and forbid
changing 'max_active' value for ordered and BH worker.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/workqueue.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 253311af47c6d..625ee8cc47f40 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -7097,6 +7097,13 @@ static ssize_t max_active_store(struct device *dev,
 	struct workqueue_struct *wq = dev_to_wq(dev);
 	int val;
 
+	/*
+	 * Adjusting max_active breaks ordering guarantee. Changing it has no
+	 * effect on BH worker.
+	 */
+	if (wq->flags & (WQ_BH | __WQ_ORDERED))
+		return -EACCES;
+
 	if (sscanf(buf, "%d", &val) != 1 || val <= 0)
 		return -EINVAL;
 
@@ -7413,13 +7420,6 @@ int workqueue_sysfs_register(struct workqueue_struct *wq)
 	struct wq_device *wq_dev;
 	int ret;
 
-	/*
-	 * Adjusting max_active breaks ordering guarantee.  Disallow exposing
-	 * ordered workqueues.
-	 */
-	if (WARN_ON(wq->flags & __WQ_ORDERED))
-		return -EINVAL;
-
 	wq->wq_dev = wq_dev = kzalloc(sizeof(*wq_dev), GFP_KERNEL);
 	if (!wq_dev)
 		return -ENOMEM;
-- 
2.51.0