linux-kernel - Re: [PATCH] workqueue: remove the unneeded cpu_relax() in __queue

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Message-ID: <20140526042311.GA14432@htj.dyndns.org>
Date:	Mon, 26 May 2014 00:23:11 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] workqueue: remove the unneeded cpu_relax() in
 __queue_work()

On Thu, May 22, 2014 at 10:21:25PM +0800, Lai Jiangshan wrote:
> On Thu, May 22, 2014 at 9:47 PM, Tejun Heo <tj@...nel.org> wrote:
> This is not busy wait, the retry and numa_pwq_tbl() guarantee that
> the retry will get a new pwq (even without cpu_relax()) as the comments says,

Yes, *eventually*.  It's not guaranteed to succeed on the immediate
next try.  This is a busy wait.

> and the refcnt of this new pwq is very very likely non-zero and
> cpu_relax() can't
> increase the probability of non-zero-refcnt. cpu_relax() is useless here.
> 
> It is different from spin_lock() or some other spin code.
> 
> it is similar to the loop of __task_rq_lock() which also guarantees progress.

No, it's not.  __task_rq_lock() *already* sees the updated value to
use for the next time.  Here, we see the old one dead and the new one
is guaranteed to show up pretty soon but we're still busy waiting for
it.

-- 
tejun
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