linux-kernel - Re: [PATCH] cpuset: mm: Reduce large amounts of memory barrier related damage v2

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Date:	Wed, 07 Mar 2012 17:05:00 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Mel Gorman <mgorman@...e.de>, David Rientjes <rientjes@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <cl@...ux.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpuset: mm: Reduce large amounts of memory barrier related
 damage v2

On Tue, 6 Mar 2012 14:54:51 -0800, Andrew Morton wrote:
>>>> -static inline void put_mems_allowed(void)
>>>> +/*
>>>> + * If this returns false, the operation that took place after get_mems_allowed
>>>> + * may have failed. It is up to the caller to retry the operation if
>>>> + * appropriate
>>>> + */
>>>> +static inline bool put_mems_allowed(unsigned int seq)
>>>>  {
>>>> -	/*
>>>> -	 * ensure that reading mems_allowed and mempolicy before reducing
>>>> -	 * mems_allowed_change_disable.
>>>> -	 *
>>>> -	 * the write-side task will know that the read-side task is still
>>>> -	 * reading mems_allowed or mempolicy, don't clears old bits in the
>>>> -	 * nodemask.
>>>> -	 */
>>>> -	smp_mb();
>>>> -	--ACCESS_ONCE(current->mems_allowed_change_disable);
>>>> +	return !read_seqcount_retry(&current->mems_allowed_seq, seq);
>>>>  }
>>>>  
>>>>  static inline void set_mems_allowed(nodemask_t nodemask)
>>>
>>> How come set_mems_allowed() still uses task_lock()?
>>>
>>
>> Consistency.
>>
>> The task_lock is taken by kernel/cpuset.c when updating
>> mems_allowed so it is taken here. That said, it is unnecessary to take
>> as the two places where set_mems_allowed is used are not going to be
>> racing. In the unlikely event that set_mems_allowed() gets another user,
>> there is no harm is leaving the task_lock as it is. It's not in a hot
>> path of any description.
> 
> But shouldn't set_mems_allowed() bump mems_allowed_seq?
> 

task_lock is also used to protect mempolicy, so ...

Thanks
Miao
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