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

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Message-ID: <20120613035647.GU22848@dastard>
Date:	Wed, 13 Jun 2012 13:56:47 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Wanpeng Li <liwp.linux@...il.com>, 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 07:21:29PM +0800, Fengguang Wu wrote:
> 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 is not a race condition - it is a lock contention condition.


> 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.

I'm not sure it will be "hardly contended". That's a global lock, so
now we'll end up with updates on different bdis contending and it's
not uncommon to see a couple of thousand processes on large machines
beating on balance_dirty_pages().  Putting a global scope lock
around such a function doesn't seem like a good solution to me.

Oh, and if you want to remove the dirty_lock from
global_update_limit(), then replacing the lock with a cmpxchg loop
will do it just fine....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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