Commit d6e022f1 authored Feb 03, 2016 by Tejun Heo

workqueue: handle NUMA_NO_NODE for unbound pool_workqueue lookup



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
874bbfe6 ("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 874bbfe6 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

parent f303fccb

kernel/workqueue.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -586,6 +586,16 @@ 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]);
		}