linux-kernel - Re: [PATCH 3/6] oom: kill younger process first

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Date:	Wed, 22 Jun 2011 16:01:35 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, caiqian@...hat.com,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 3/6] oom: kill younger process first

On Wed, 22 Jun 2011, KOSAKI Motohiro wrote:

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index e4e6d7b..392ff30 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2257,6 +2257,9 @@ static inline unsigned long wait_task_inactive(struct task_struct *p,
>  #define next_task(p) \
>  	list_entry_rcu((p)->tasks.next, struct task_struct, tasks)
> 
> +#define prev_task(p) \
> +	list_entry((p)->tasks.prev, struct task_struct, tasks)
> +
>  #define for_each_process(p) \
>  	for (p = &init_task ; (p = next_task(p)) != &init_task ; )
> 
> @@ -2269,6 +2272,14 @@ extern bool current_is_single_threaded(void);
>  #define do_each_thread(g, t) \
>  	for (g = t = &init_task ; (g = t = next_task(g)) != &init_task ; ) do
> 
> +/*
> + * Similar to do_each_thread(). but two difference are there.
> + *  - traverse tasks reverse order (i.e. younger to older)
> + *  - caller must hold tasklist_lock. rcu_read_lock isn't enough
> +*/
> +#define do_each_thread_reverse(g, t) \
> +	for (g = t = &init_task ; (g = t = prev_task(g)) != &init_task ; ) do
> +
>  #define while_each_thread(g, t) \
>  	while ((t = next_thread(t)) != g)
> 

I've already ack'd the patch, but if there is another version posted as a 
result of our discussion of using euid in the heuristic, I think it would 
be helpful to reiterate in this comment that, like do_each_thread(), 
do_each_thread_reverse() is not break-safe either.  It might end up 
preventing a bug down the road.
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