linux-kernel - Re: [PATCH 00/21] workqueue: cleanups and better locking for recent changes

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Date:	Wed, 20 Mar 2013 11:30:37 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/21] workqueue: cleanups and better locking for recent
 changes

So, overall,

On Wed, Mar 20, 2013 at 03:28:00AM +0800, Lai Jiangshan wrote:
...
> In this list, we can find that:
> 1) wq_mutex protects too much different kind of things.

I don't agree with this and unless you can come up with a much better
reason, won't be splitting wq_mutex further.  Also, I'm not gonna
rename it to wqs_mutex.

> 2) workqueue->pwqs are protected by both wq->flush_mutex and pwq_lock,
>    but in many read sites, they are protected by both wq->flush_mutex and pwq_lock,
>    in some other read sites, they are protected by pwq_lock, but can be converted
>    to wq->flush_mutex. it means pwq_lock and wq->flush_mutex are redundancy here.
> 3) pwq_lock is global lock, but it protects only workqueue instance fields.

A global lock protecting different instances is perfectly fine unless
it actually causes contention in some paths.  It often actually is
better to have a single global lock for cold paths as it pollutes less
amount of cache, so please don't split locks for that reason.

That said, I like the fact that wq->flush_mutex can be transformed
into wq->mutex and replaces pwq_lock making it go away, so, yeah, for
that reason, I like pwq_lock removal, so yeah, let's replace pwq_lock
with wq->mutex.

I applied the ones I can apply at this point.  I'll rebase the NUMA
patchset on top of the applied ones.  Please update the rest on top.

Thanks.

-- 
tejun
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