From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 09/27] nfs: writeback pages wait queue
Date: Thu, 03 Mar 2011 17:08:01 +0100
Message-ID: <1299168481.1310.56.camel@laptop>
References: <20110303064505.718671603@intel.com>
	 <20110303074949.809203319@intel.com>
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Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
	Jens Axboe <axboe@kernel.dk>,
	Chris Mason <chris.mason@oracle.com>,
	Trond Myklebust <Trond.Myklebust@netapp.com>,
	Christoph Hellwig <hch@lst.de>,
	Dave Chinner <david@fromorbit.com>,
	Theodore Ts'o <tytso@mit.edu>, Mel Gorman <mel@csn.ul.ie>,
	Rik van Riel <riel@redhat.com>,
	KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
	Greg Thelen <gthelen@google.com>,
	Minchan Kim <minchan.kim@gmail.com>,
	Vivek Goyal <vgoyal@redhat.com>,
	Andrea Righi <arighi@develer.com>,
	Balbir Singh <balbir@linux.vnet.ibm.com>,
	linux-mm <linux-mm@kvack.org>, linux-fsdevel@vger.kernel.org,
	LKML <linux-kernel@vger.kernel.org>
To: Wu Fengguang <fengguang.wu@intel.com>
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On Thu, 2011-03-03 at 14:45 +0800, Wu Fengguang wrote:
> +static void nfs_wakeup_congested(long nr,
> +                                struct backing_dev_info *bdi,
> +                                wait_queue_head_t *wqh)
> +{
> +       long limit = nfs_congestion_kb >> (PAGE_SHIFT - 10);
> +
> +       if (nr < 2 * limit - min(limit / 8, NFS_WAIT_PAGES)) {
> +               if (test_bit(BDI_sync_congested, &bdi->state)) {
> +                       clear_bdi_congested(bdi, BLK_RW_SYNC);
> +                       smp_mb__after_clear_bit();
> +               }
> +               if (waitqueue_active(&wqh[BLK_RW_SYNC]))
> +                       wake_up(&wqh[BLK_RW_SYNC]);
> +       }
> +       if (nr < limit - min(limit / 8, NFS_WAIT_PAGES)) {
> +               if (test_bit(BDI_async_congested, &bdi->state)) {
> +                       clear_bdi_congested(bdi, BLK_RW_ASYNC);
> +                       smp_mb__after_clear_bit();
> +               }
> +               if (waitqueue_active(&wqh[BLK_RW_ASYNC]))
> +                       wake_up(&wqh[BLK_RW_ASYNC]);
> +       }
> +} 

memory barriers want a comment - always - explaining what they order and
against whoem.