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

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Date:	Thu, 03 Mar 2011 17:08:01 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/27] nfs: writeback pages wait queue

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.

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