linux-kernel - Re: [PATCH 6/6] cpuset dirty limits

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Message-ID: <46F06C17.5050203@google.com>
Date:	Tue, 18 Sep 2007 17:23:51 -0700
From:	Ethan Solomita <solo@...gle.com>
To:	Christoph Lameter <clameter@....com>
CC:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	pj@....com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] cpuset dirty limits

Christoph Lameter wrote:
> On Fri, 14 Sep 2007, Andrew Morton wrote:
> 
>>> +	mutex_lock(&callback_mutex);
>>> +	*cs_int = val;
>>> +	mutex_unlock(&callback_mutex);
>> I don't think this locking does anything?
> 
> Locking is wrong here. The lock needs to be taken before the cs pointer 
> is dereferenced from the caller.

	I think we can just remove the callback_mutex lock. Since the change is
coming from an update to a cpuset filesystem file, the cpuset is not
going anywhere since the inode is open. And I don't see that any code
really cares whether the dirty ratios change out from under them.

> 
>>> +	return 0;
>>> +}
>>> +
>>>  /*
>>>   * Frequency meter - How fast is some event occurring?
>>>   *
>>> ...
>>> +void cpuset_get_current_ratios(int *background_ratio, int *throttle_ratio)
>>> +{
>>> +	int background = -1;
>>> +	int throttle = -1;
>>> +	struct task_struct *tsk = current;
>>> +
>>> +	task_lock(tsk);
>>> +	background = task_cs(tsk)->background_dirty_ratio;
>>> +	throttle = task_cs(tsk)->throttle_dirty_ratio;
>>> +	task_unlock(tsk);
>> ditto?
> 
> It is required to take the task lock while dereferencing the tasks cpuset 
> pointer.

	Agreed.
	-- Ethan
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