linux-kernel - Re: [PATCH 09/27] nfs: writeback pages wait queue

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Date:	Fri, 04 Mar 2011 10:26:35 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Jens Axboe <axboe@...nel.dk>,
	Chris Mason <chris.mason@...cle.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>, Mel Gorman <mel@....ul.ie>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/27] nfs: writeback pages wait queue

On Fri, 2011-03-04 at 10:10 +0100, Peter Zijlstra wrote:
> On Fri, 2011-03-04 at 10:01 +0800, Wu Fengguang wrote:
> >                         clear_bdi_congested(bdi, BLK_RW_SYNC);
> >                         /*
> >                          * On the following wake_up(), nfs_wait_congested()
> >                          * will see the cleared bit and quit.
> >                          */
> >                         smp_mb__after_clear_bit();
> >                 }
> >                 if (waitqueue_active(&wqh[BLK_RW_SYNC]))
> >                         wake_up(&wqh[BLK_RW_SYNC]); 
> 
> If I tell you that: try_to_wake_up() implies an smp_wmb(), do you then
> still need this?

Also, there is no matching rmb,mb in nfs_wait_congested().. barrier
always come in pairs.
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