3.18-stable review patch. If anyone has any objections, please let me know.
------------------ From: Tejun Heo <t...@kernel.org> commit 5c0338c68706be53b3dc472e4308961c36e4ece1 upstream. The combination of WQ_UNBOUND and max_active == 1 used to imply ordered execution. After NUMA affinity 4c16bd327c74 ("workqueue: implement NUMA affinity for unbound workqueues"), this is no longer true due to per-node worker pools. While the right way to create an ordered workqueue is alloc_ordered_workqueue(), the documentation has been misleading for a long time and people do use WQ_UNBOUND and max_active == 1 for ordered workqueues which can lead to subtle bugs which are very difficult to trigger. It's unlikely that we'd see noticeable performance impact by enforcing ordering on WQ_UNBOUND / max_active == 1 workqueues. Let's automatically set __WQ_ORDERED for those workqueues. Signed-off-by: Tejun Heo <t...@kernel.org> Reported-by: Christoph Hellwig <h...@infradead.org> Reported-by: Alexei Potashnik <ale...@purestorage.com> Fixes: 4c16bd327c74 ("workqueue: implement NUMA affinity for unbound workqueues") Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- kernel/workqueue.c | 10 ++++++++++ 1 file changed, 10 insertions(+) --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -4075,6 +4075,16 @@ struct workqueue_struct *__alloc_workque struct workqueue_struct *wq; struct pool_workqueue *pwq; + /* + * Unbound && max_active == 1 used to imply ordered, which is no + * longer the case on NUMA machines due to per-node pools. While + * alloc_ordered_workqueue() is the right way to create an ordered + * workqueue, keep the previous behavior to avoid subtle breakages + * on NUMA. + */ + if ((flags & WQ_UNBOUND) && max_active == 1) + flags |= __WQ_ORDERED; + /* see the comment above the definition of WQ_POWER_EFFICIENT */ if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient) flags |= WQ_UNBOUND;