workqueue: handle NUMA_NO_NODE for unbound pool_workqueue lookup

Orabug: 23330855

[ Upstream commit d6e022f1d207a161cd88e08ef0371554680ffc46 ]

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@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
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
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Dan Duval <dan.duval@oracle.com>
(cherry picked from f3d69fd89df665c1fa1931f4af846039a4ac5dc4)

Conflict:

	kernel/workqueue.c

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5c01664c26e25e3e998c1d537093dd3832032a84..5e5cbd834fb73312e67c8efcdb95a0b1f0929dcb 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -560,7 +560,17 @@ static int worker_pool_assign_id(struct worker_pool *pool)
 static struct pool_workqueue *unbound_pwq_by_node(struct workqueue_struct *wq,
                                                  int node)
 {
-       assert_rcu_or_wq_mutex(wq);
+       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]);
 }