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Subject: Re: [PATCH 09/27] nfs: writeback pages wait queue
From: Peter Zijlstra <peterz@infradead.org>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
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Date: Fri, 04 Mar 2011 10:26:35 +0100
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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.