Patch "workqueue: restore WQ_UNBOUND/max_active==1 to be ordered" has been added to the 4.12-stable tree

This is a note to let you know that I've just added the patch titled

    workqueue: restore WQ_UNBOUND/max_active==1 to be ordered

to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
workqueue-restore-wq_unbound-max_active-1-to-be-ordered.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Tue, 18 Jul 2017 18:41:52 -0400
Subject: workqueue: restore WQ_UNBOUND/max_active==1 to be ordered

From: Tejun Heo <[email protected]>

commit 5c0338c68706be53b3dc472e4308961c36e4ece1 upstream.

The combination of WQ_UNBOUND and max_active == 1 used to imply ordered execution. After NUMA affinity 4c16bd327c74 ("workqueue:
implement NUMA affinity for unbound workqueues"), this is no longer true due to per-node worker pools.

While the right way to create an ordered workqueue is alloc_ordered_workqueue(), the documentation has been misleading for a long time and people do use WQ_UNBOUND and max_active == 1 for ordered workqueues which can lead to subtle bugs which are very difficult to trigger.

It's unlikely that we'd see noticeable performance impact by enforcing ordering on WQ_UNBOUND / max_active == 1 workqueues. Let's automatically set __WQ_ORDERED for those workqueues.

---
 kernel/workqueue.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

@@ -3929,6 +3929,16 @@ struct workqueue_struct *__alloc_workque
 	struct workqueue_struct *wq;
 	struct pool_workqueue *pwq;
 
+	/*
+	 * Unbound && max_active == 1 used to imply ordered, which is no
+	 * longer the case on NUMA machines due to per-node pools.  While
+	 * alloc_ordered_workqueue() is the right way to create an ordered
+	 * workqueue, keep the previous behavior to avoid subtle breakages
+	 * on NUMA.
+	 */
+	if ((flags & WQ_UNBOUND) && max_active == 1)
+		flags |= __WQ_ORDERED;
+
 	/* see the comment above the definition of WQ_POWER_EFFICIENT */
 	if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient)
 		flags |= WQ_UNBOUND;

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/cgroup-don-t-call-migration-methods-if-there-are-no-tasks-to-migrate.patch queue-4.12/cgroup-create-dfl_root-files-on-subsys-registration.patch queue-4.12/cgroup-fix-error-return-value-from-cgroup_subtree_control.patch queue-4.12/workqueue-restore-wq_unbound-max_active-1-to-be-ordered.patch queue-4.12/libata-array-underflow-in-ata_find_dev.patch