On Wed, 3 Feb 2016, Tejun Heo wrote:
> 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 <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 # v4.3+
4.3+ ? Hasn't 874bbfe600a6 been backported to older stable kernels?
Adding a 'Fixes: 874bbfe600a6 ...' tag is what you really want here.
> 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.
So what happens if the complete node is offline?
> + */
> + if (unlikely(node == NUMA_NO_NODE))
> + return wq->dfl_pwq;
> +
Thanks,
tglx
