linux-kernel - Re: [PATCH 1/1] [RFC] workqueue: fix ghost PENDING flag while doing MQ IO

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Date:	Wed, 27 Apr 2016 07:50:01 +0200
From:	Hannes Reinecke <hare@...e.de>
To:	Tejun Heo <tj@...nel.org>, Peter Hurley <peter@...leysoftware.com>
Cc:	Roman Pen <roman.penyaev@...fitbricks.com>,
	Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 1/1] [RFC] workqueue: fix ghost PENDING flag while doing
 MQ IO

On 04/26/2016 07:45 PM, Tejun Heo wrote:
> Hello, Peter.
> 
> On Tue, Apr 26, 2016 at 10:27:59AM -0700, Peter Hurley wrote:
>>> It's unlikely to make any measureable difference.  Is xchg() actually
>>> cheaper than store + rmb?
>>
>> store + mfence (full barrier), yes. Roughly 2x faster.
>>
>> https://lkml.org/lkml/2015/11/2/607
> 
> Ah, didn't know that.  Thanks for the pointer.
> 
>>> I'm not necessarily against making all clearings of
>>> PENDING to be followed by a rmb or use xhcg.  Reasons 2-4 are pretty
>>> weak tho.
>>
>> I agree 2 and 3 are not the best reasons.
>> Actually, it looks that I'm in the minority anyway, and that style-wise,
>> naked barrier is preferred.
> 
> As long as what's happening is clearly documented, I think either is
> fine.  I'm gonna go with Roman's mb patch for -stable fix but think
> it'd be nice to have a separate patch to consolidate the paths which
> clear PENDING and make them use xchg.  If you can spin up a patch for
> that, I'd be happy to apply it to wq/for-3.7.
>                                          ^^^
Ah. Time warp.
I knew it would happen eventually :-)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@...e.de			               +49 911 74053 688
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