linux-kernel - [PATCH 3.16.y-ckt 115/129] workqueue: handle NUMA_NO_NODE for unbound pool

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Message-Id: <1456482100-4667-116-git-send-email-luis.henriques@canonical.com>
Date:	Fri, 26 Feb 2016 10:21:26 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Tejun Heo <tj@...nel.org>, Tang Chen <tangchen@...fujitsu.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Len Brown <len.brown@...el.com>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 115/129] workqueue: handle NUMA_NO_NODE for unbound pool_workqueue lookup

3.16.7-ckt25 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Tejun Heo <tj@...nel.org>

commit d6e022f1d207a161cd88e08ef0371554680ffc46 upstream.

When looking up the pool_workqueue to use for an unbound workqueue,
workqueue assumes that the target CPU is always bound to a valid NUMA
node.  However, currently, when a CPU goes offline, the mapping is
destroyed and cpu_to_node() returns NUMA_NO_NODE.

This has always been broken but hasn't triggered often enough before
874bbfe600a6 ("workqueue: make sure delayed work run in local cpu").
After the commit, workqueue forcifully assigns the local CPU for
delayed work items without explicit target CPU to fix a different
issue.  This widens the window where CPU can go offline while a
delayed work item is pending causing delayed work items dispatched
with target CPU set to an already offlined CPU.  The resulting
NUMA_NO_NODE mapping makes workqueue try to queue the work item on a
NULL pool_workqueue and thus crash.

While 874bbfe600a6 has been reverted for a different reason making the
bug less visible again, it can still happen.  Fix it by mapping
NUMA_NO_NODE to the default pool_workqueue from unbound_pwq_by_node().
This is a temporary workaround.  The long term solution is keeping CPU
-> NODE mapping stable across CPU off/online cycles which is being
worked on.

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Tang Chen <tangchen@...fujitsu.com>
Cc: Rafael J. Wysocki <rafael@...nel.org>
Cc: Len Brown <len.brown@...el.com>
Link: http://lkml.kernel.org/g/1454424264.11183.46.camel@gmail.com
Link: http://lkml.kernel.org/g/1453702100-2597-1-git-send-email-tangchen@cn.fujitsu.com
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 kernel/workqueue.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index cb7db323d1fb..6ab1f683ac49 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -553,6 +553,16 @@ static struct pool_workqueue *unbound_pwq_by_node(struct workqueue_struct *wq,
 						  int node)
 {
 	assert_rcu_or_wq_mutex(wq);
+
+	/*
+	 * XXX: @node can be NUMA_NO_NODE if CPU goes offline while a
+	 * delayed item is pending.  The plan is to keep CPU -> NODE
+	 * mapping valid and stable across CPU on/offlines.  Once that
+	 * happens, this workaround can be removed.
+	 */
+	if (unlikely(node == NUMA_NO_NODE))
+		return wq->dfl_pwq;
+
 	return rcu_dereference_raw(wq->numa_pwq_tbl[node]);
 }