Subject: Re: [PATCH 1/1] [RFC] workqueue: fix ghost PENDING flag while doing
 MQ IO
To: Tejun Heo <tj@kernel.org>, Peter Hurley <peter@hurleysoftware.com>
References: <1461597771-25352-1-git-send-email-roman.penyaev@profitbricks.com>
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 <20160426151507.GK7822@mtj.duckdns.org> <571FA51F.2040702@hurleysoftware.com>
 <20160426174513.GM7822@mtj.duckdns.org>
Cc: Roman Pen <roman.penyaev@profitbricks.com>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        David Howells <dhowells@redhat.com>
From: Hannes Reinecke <hare@suse.de>
Message-ID: <57205309.2060708@suse.de>
Date: Wed, 27 Apr 2016 07:50:01 +0200
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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@suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 N?rnberg
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