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

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

on 2010-3-5 20:03, Paul Menage wrote:
> On Wed, Mar 3, 2010 at 2:52 AM, Miao Xie <miaox@cn.fujitsu.com> wrote:
>> if MAX_NUMNODES > BITS_PER_LONG, loading/storing task->mems_allowed or mems_allowed in
>> task->mempolicy are not atomic operations, and the kernel page allocator gets an empty
>> mems_allowed when updating task->mems_allowed or mems_allowed in task->mempolicy. So we
>> use a rwlock to protect them to fix this probelm.
> 
> Rather than adding locks, if the intention is just to avoid the
> allocator seeing an empty nodemask couldn't we instead do the
> equivalent of:
> 
> current->mems_allowed |= new_mask;
> current->mems_allowed = new_mask;
> 
> i.e. effectively set all new bits in the nodemask first, and then
> clear all old bits that are no longer in the new mask. The only
> downside of this is that a page allocation that races with the update
> could potentially allocate from any node in the union of the old and
> new nodemasks - but that's the case anyway for an allocation that
> races with an update, so I don't see that it's any worse.

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.

I make a new patch to fix this problem now.
Considering the change of task->mems_allowed is not frequent, so in the new
patch, I use variables as a tag to indicate whether task->mems_allowed need
be update or not. And before setting the tag, cpuset caches the new mask of
every task at somewhere. 

When the allocator want to access task->mems_allowed, it must check updated-tag
first. If the tag is set, the allocator enters the slow path and updates
task->mems_allowed.

Thanks!
Miao

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