Commit 5c0338c6 authored Jul 18, 2017 by Tejun Heo

workqueue: restore WQ_UNBOUND/max_active==1 to be ordered



The combination of WQ_UNBOUND and max_active == 1 used to imply
ordered execution.  After NUMA affinity 4c16bd32 ("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 <tj@kernel.org>
Reported-by: Christoph Hellwig <hch@infradead.org>
Reported-by: Alexei Potashnik <alexei@purestorage.com>
Fixes: 4c16bd32 ("workqueue: implement NUMA affinity for unbound workqueues")
Cc: stable@vger.kernel.org # v3.10+

parent 74cbd96b

kernel/workqueue.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -3929,6 +3929,16 @@ struct workqueue_struct __alloc_workqueue_key(const char fmt,
		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;

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