Re: [PATCH 2/4] oom: kill younger process first

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, May 12, 2011 at 12:39 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Thu, 12 May 2011 11:23:38 +0900
> Minchan Kim <minchan.kim@gmail.com> wrote:
>
>> On Thu, May 12, 2011 at 10:53 AM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> > On Thu, 12 May 2011 10:30:45 +0900
>> > Minchan Kim <minchan.kim@gmail.com> wrote:
>
>> > As above implies, (B)->prev pointer is invalid pointer after list_del().
>> > So, there will be race with list modification and for_each_list_reverse under
>> > rcu_read__lock()
>> >
>> > So, when you need to take atomic lock (as tasklist lock is) is...
>> >
>> >  1) You can't check 'entry' is valid or not...
>> >    In above for_each_list_rcu(), you may visit an object which is under removing.
>> >    You need some flag or check to see the object is valid or not.
>> >
>> >  2) you want to use list_for_each_safe().
>> >    You can't do list_del() an object which is under removing...
>> >
>> >  3) You want to walk the list in reverse.
>> >
>> >  3) Some other reasons. For example, you'll access an object pointed by the
>> >    'entry' and the object is not rcu safe.
>> >
>> > make sense ?
>>
>> Yes. Thanks, Kame.
>> It seems It is caused by prev poisoning of list_del_rcu.
>> If we remove it, isn't it possible to traverse reverse without atomic lock?
>>
>
> IIUC, it's possible (Fix me if I'm wrong) but I don't like that because of 2 reasons.
>
> 1. LIST_POISON is very important information at debug.

Indeed.
But if we can get a better something although we lost debug facility,
I think it would be okay.

>
> 2. If we don't clear prev pointer, ok, we'll allow 2 directional walk of list
>   under RCU.
>   But, in following case
>   1. you are now at (C). you'll visit (C)->next...(D)
>   2. you are now at (D). you want to go back to (C) via (D)->prev.
>   3. But (D)->prev points to (B)
>
>  It's not a 2 directional list, something other or broken one.

Yes. but it shouldn't be a problem in RCU semantics.
If you need such consistency, you should use lock.

I recall old thread about it.
In http://lwn.net/Articles/262464/, mmutz and Paul already discussed
about it. :)

>  Then, the rculist is 1 directional list in nature, I think.

Yes. But Why RCU become 1 directional list is we can't find a useful usecases.

>
> So, without very very big reason, we should keep POISON.

Agree.
I don't insist on it as it's not a useful usecase for persuading Paul.
That's because it's not a hot path.

It's started from just out of curiosity.
Thanks for very much clarifying that, Kame!

-- 
Kind regards,
Minchan Kim

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