From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 20/45] NFS: introduce writeback wait queue
Date: Wed, 07 Oct 2009 11:15:36 +0200
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Cc: Andrew Morton <akpm@linux-foundation.org>,
	Theodore Tso <tytso@mit.edu>,
	Christoph Hellwig <hch@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Chris Mason <chris.mason@oracle.com>,
	"Li, Shaohua" <shaohua.li@intel.com>,
	Myklebust Trond <Trond.Myklebust@netapp.com>,
	"jens.axboe@oracle.com" <jens.axboe@oracle.com>,
	Jan Kara <jack@suse.cz>, Nick Piggin <npiggin@suse.de>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
To: Wu Fengguang <fengguang.wu@intel.com>
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On Wed, 2009-10-07 at 17:07 +0800, Wu Fengguang wrote:
> 
> > wakeup implies a full memory barrier.
> 
> If so, this smp_mb__after_clear_bit() line is also not necessary?
> 
> void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
> {
> //...
>         clear_bit(bit, &bdi->state);
>         smp_mb__after_clear_bit();
>         if (waitqueue_active(wqh))
>                 wake_up(wqh);

Depends on if the barrier is needed even when the wakeup doesn't happen
I guess ;-)