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

[Miao Xie <miaox@cn.fujitsu.com>]

on 2010-3-10 3:42, Paul Menage wrote:
> On Sat, Mar 6, 2010 at 6:33 PM, Miao Xie <miaox@cn.fujitsu.com> wrote:
>>
>> Before applying this patch, cpuset updates task->mems_allowed just like
>> what you said. But the allocator is still likely to see an empty nodemask.
>> This problem have been pointed out by Nick Piggin.
>>
>> 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.

Thanks!
Miao

> 
> Paul
> 
> 
>