linux-kernel - Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy and mems

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Message-ID: <4B98A263.8030903@cn.fujitsu.com>
Date:	Thu, 11 Mar 2010 15:57:23 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	Nick Piggin <npiggin@...e.de>
CC:	Paul Menage <menage@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy
 and 	mems_allowed

on 2010-3-11 13:30, Nick Piggin wrote:
>>>> The problem is following:
>>>> The size of nodemask_t is greater than the size of long integer, so loading
>>>> and storing of nodemask_t are not atomic operations. If task->mems_allowed
>>>> don't intersect with new_mask, such as the first word of the mask is empty
>>>> and only the first word of new_mask is not empty. When the allocator
>>>> loads a word of the mask before
>>>>
>>>>        current->mems_allowed |= new_mask;
>>>>
>>>> and then loads another word of the mask after
>>>>
>>>>        current->mems_allowed = new_mask;
>>>>
>>>> the allocator gets an empty nodemask.
>>>
>>> Couldn't that be solved by having the reader read the nodemask twice
>>> and compare them? In the normal case there's no race, so the second
>>> read is straight from L1 cache and is very cheap. In the unlikely case
>>> of a race, the reader would keep trying until it got two consistent
>>> values in a row.
>>
>> I think this method can't fix the problem because we can guarantee the second
>> read is after the update of mask completes.
> 
> Any problem with using a seqlock?
> 
> The other thing you could do is store a pointer to the nodemask, and
> allocate a new nodemask when changing it, issue a smp_wmb(), and then
> store the new pointer. Read side only needs a smp_read_barrier_depends()

Comparing with my second version patch, I think both of these methods will cause worse
performance and the changing of code is more.

Thanks
Miao

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