linux-kernel - [PATCH 3.10 040/139] workqueue: restore WQ_UNBOUND/max

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Message-Id: <1509571159-4405-41-git-send-email-w@1wt.eu>
Date:   Wed,  1 Nov 2017 22:17:40 +0100
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Tejun Heo <tj@...nel.org>, Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 040/139] workqueue: restore WQ_UNBOUND/max_active==1 to be ordered

From: Tejun Heo <tj@...nel.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@...nel.org>
Reported-by: Christoph Hellwig <hch@...radead.org>
Reported-by: Alexei Potashnik <alexei@...estorage.com>
Fixes: 4c16bd327c74 ("workqueue: implement NUMA affinity for unbound workqueues")
Cc: stable@...r.kernel.org 
Signed-off-by: Willy Tarreau <w@....eu>
---
 kernel/workqueue.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 66972ac..3f8558f 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4213,6 +4213,16 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
 	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;
+
 	/* allocate wq and format name */
 	if (flags & WQ_UNBOUND)
 		tbl_size = wq_numa_tbl_len * sizeof(wq->numa_pwq_tbl[0]);
-- 
2.8.0.rc2.1.gbe9624a