Re: [PATCH 1/4] cpuset: fix the problem that cpuset_mem_spread_node() returns an offline node(was: Re: [regression] cpuset,mm: update tasks' mems_allowed in time (58568d2))

Miao Xie
2010-03-04T09:31:42Z

on 2010-3-4 11:22, Nick Piggin wrote:
...
>> + /*
>> + * After current->mems_allowed is set to a new value, current will
>> + * allocate new pages for the migrating memory region. So we must
>> + * ensure that update of current->mems_allowed have been completed
>> + * by this moment.
>> + */
>> + smp_wmb();
>> do_migrate_pages(mm, from, to, MPOL_MF_MOVE_ALL);
>>
>> guarantee_online_mems(task_cs(tsk),&tsk->mems_allowed);
>> +
>> + /*
>> + * After doing migrate pages, current will allocate new pages for
>> + * itself not the other tasks. So we must ensure that update of
>> + * current->mems_allowed have been completed by this moment.
>> + */
>> + smp_wmb();
>
> The comments don't really make sense. A task always sees its own
> memory operations in program order. You keep saying *current* allocates
> pages so *current*->mems_allowed must be updated. This doesn't make
> sense. Do you mean to say tsk->?
>
> Secondly, memory ordering operations do not ensure anything is
> completed. They only ensure ordering. So to make sense to use them,
> you generally need corresponding barriers in other code that can
> run concurrently.
>
> So you need to comment what is being ordered (ie. at least 2 memory
> operations). And what other code might be running that requires this
> ordering.
>
> You need to comment to all these sites and operations. Sprinkling of
> memory barriers just gets unmaintainable.

My thought is wrong.
I thought the kernel might call do_migrate_pages() before updating
->mems_allowed, so I used smp_wmb() to ensure this order.

In fact, this problem which I worried can't occur, so these smp_wmb()
is unnecessary.

Thanks!
Miao