linux-kernel - Re: [PATCH wq/for-4.13-fixes] workqueue: restore WQ_UNBOUND/max

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Message-ID: <20170719152557.GL3365493@devbig577.frc2.facebook.com>
Date:   Wed, 19 Jul 2017 11:25:57 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alexei Potashnik <alexei@...estorage.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH wq/for-4.13-fixes] workqueue: restore
 WQ_UNBOUND/max_active==1 to be ordered

On Tue, Jul 18, 2017 at 06:41:52PM -0400, Tejun Heo wrote:
> 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 <tj@...nel.org>
> Reported-by: Christoph Hellwig <hch@...radead.org>
> Reported-by: Alexei Potashnik <alexei@...estorage.com>
> Fixes: 4c16bd327c74 ("workqueue: implement NUMA affinity for unbound workqueues")
> Cc: stable@...r.kernel.org # v3.10+

Applied to wq/for-4.13-fixes.

Thanks.

-- 
tejun