Patch "workqueue: restore WQ_UNBOUND/max_active==1 to be ordered" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

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 4.4-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-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 5c0338c68706be53b3dc472e4308961c36e4ece1 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Tue, 18 Jul 2017 18:41:52 -0400
Subject: workqueue: restore WQ_UNBOUND/max_active==1 to be ordered

From: Tejun Heo <tj@kernel.org>

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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Christoph Hellwig <hch@infradead.org>
Reported-by: Alexei Potashnik <alexei@purestorage.com>
Fixes: 4c16bd327c74 ("workqueue: implement NUMA affinity for unbound workqueues")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3834,6 +3834,16 @@ 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 tj@kernel.org are

queue-4.4/workqueue-restore-wq_unbound-max_active-1-to-be-ordered.patch
queue-4.4/libata-array-underflow-in-ata_find_dev.patch