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

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Message-ID: <20160426152156.GL7822@mtj.duckdns.org>
Date:	Tue, 26 Apr 2016 11:21:56 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Roman Pen <roman.penyaev@...fitbricks.com>
Cc:	Gioh Kim <gi-oh.kim@...fitbricks.com>,
	Michael Wang <yun.wang@...fitbricks.com>,
	Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/1] workqueue: fix ghost PENDING flag while doing MQ IO

Hello,

On Tue, Apr 26, 2016 at 01:15:35PM +0200, Roman Pen wrote:
...
> This behaviour can be explained by speculative LOAD of hctx->ctx_map on
> CPU#0, which is reordered with clear of PENDING bit and executed _before_
> actual STORE of bit 1 on CPU#1.
> 
> The proper fix is an explicit full barrier <mfence>, which guarantees
> that clear of PENDING bit is to be executed before all possible
> speculative LOADS or STORES inside actual work function.

Applied to workqueue/for-4.6-fixes.

Thanks.

-- 
tejun