linux-kernel - Re: [PATCH] writeback: avoid race when update bandwidth

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Message-ID: <20120612112129.GA16639@localhost>
Date:	Tue, 12 Jun 2012 19:21:29 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Wanpeng Li <liwp.linux@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Gavin Shan <shangw@...ux.vnet.ibm.com>,
	Wanpeng Li <liswp@...ux.vnet.ibm.com>
Subject: Re: [PATCH] writeback: avoid race when update bandwidth

On Tue, Jun 12, 2012 at 06:26:43PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <liwp@...ux.vnet.ibm.com>

That email address is no longer in use?

> Since bdi->wb.list_lock is used to protect the b_* lists,
> so the flushers who call wb_writeback to writeback pages will
> stuck when bandwidth update policy holds this lock. In order
> to avoid this race we can introduce a new bandwidth_lock who
> is responsible for protecting bandwidth update policy.

This looks good to me. wb.list_lock could be contended and it's better
for bdi_update_bandwidth() to use a standalone and hardly contended
lock.

btw, with this change, the dirty_lock in global_update_bandwidth() can
be eliminated.

> Signed-off-by: Wanpeng Li <liswp@...ux.vnet.ibm.com>
> ---
>  mm/page-writeback.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index c8945e0..b3b08fb 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1032,12 +1032,14 @@ static void bdi_update_bandwidth(struct backing_dev_info *bdi,
>  				 unsigned long bdi_dirty,
>  				 unsigned long start_time)
>  {
> +	static DEFINE_SPINLOCK(bandwidth_lock);
> +
>  	if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
>  		return;
> -	spin_lock(&bdi->wb.list_lock);
> +	spin_lock(&bandwidth_lock);
>  	__bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
>  			       bdi_thresh, bdi_dirty, start_time);
> -	spin_unlock(&bdi->wb.list_lock);
> +	spin_unlock(&bandwidth_lock);
>  }
>  
>  /*
> -- 
> 1.7.9.5
--
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