linux-kernel - Re: [PATCH 5/7 V2] workqueue: reuse the current default pwq when its attrs unchanged

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Message-ID: <20150518203809.GF24861@htj.duckdns.org>
Date:	Mon, 18 May 2015 16:38:09 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/7 V2] workqueue: reuse the current default pwq when its
 attrs unchanged

Hello,

On Tue, May 12, 2015 at 08:32:33PM +0800, Lai Jiangshan wrote:
> When apply_wqattrs_prepare() is called, it is possible that the default
> pwq is unaffected.  It is always true that only the NUMA affinity is being
> changed and sometimes true that the low level cpumask is being changed.
> 
> So we try to reuse the current default pwq when its attrs unchanged.
> 
> After this change, "ctx->dfl_pwq->refcnt++" could be dangerous
> when ctx->dfl_pwq is being reused, so we use get_pwq_unlocked() instead.

Can you elaborate the above paragraph?  Why wouldn't that be dangerous
before this change?

Thanks.

-- 
tejun
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