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 <[email protected]>
Reported-by: Mike Galbraith <[email protected]>
Cc: Tang Chen <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: Len Brown <[email protected]>
Cc: [email protected] # v4.3+
Fixes: 874bbfe600a6 ("workqueue: make sure delayed work run in local cpu")
Link: http://lkml.kernel.org/g/[email protected]
Link:
http://lkml.kernel.org/g/[email protected]
---
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]);
}
