[PATCH wq/for-4.5-fixes] workqueue: handle NUMA_NO_NODE for unbound pool_workqueue lookup

[Tejun Heo <tj@kernel.org>]

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 until recently.

After 874bbfe600a6 ("workqueue: make sure delayed work run in local
cpu"), 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.

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 in the works.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Len Brown <len.brown@intel.com>
Cc: stable@vger.kernel.org # v4.3+
Fixes: 874bbfe600a6 ("workqueue: make sure delayed work run in local cpu")
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
---
 kernel/workqueue.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 61a0264..f748eab 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -570,6 +570,16 @@ static struct pool_workqueue *unbound_pwq_by_node(struct workqueue_struct *wq,
 						  int node)
 {
 	assert_rcu_or_wq_mutex_or_pool_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]);
 }