Hello,
With 874bbfe600a6 reverted, this doesn't matter as much but I'm still
committing it to wq/for-4.5-fixes. The commit message has been
updated accordingly.
Thanks.
------ 8< ------
>From 9354baa208f47bf088c28c1da5f38abe172db5d6 Mon Sep 17 00:00:00 2001
From: Tejun Heo <t...@kernel.org>
Date: Wed, 3 Feb 2016 13:54:25 -0500
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 <t...@kernel.org>
Reported-by: Mike Galbraith <umgwanakikb...@gmail.com>
Cc: Tang Chen <tangc...@cn.fujitsu.com>
Cc: Rafael J. Wysocki <raf...@kernel.org>
Cc: Len Brown <len.br...@intel.com>
Cc: sta...@vger.kernel.org
Link: http://lkml.kernel.org/g/1454424264.11183.46.ca...@gmail.com
Link:
http://lkml.kernel.org/g/1453702100-2597-1-git-send-email-tangc...@cn.fujitsu.com
---
kernel/workqueue.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index dc7faad..3ca7ab8 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]);
}
--
2.5.0
