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Subject: Re: [PATCH 09/27] nfs: writeback pages wait queue
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
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Date: Thu, 03 Mar 2011 17:08:01 +0100
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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.