linux-kernel - Re: [PATCH 3/4] mm, oom: do not rely on TIF_MEMDIE for exit_oom

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Date:   Tue, 4 Oct 2016 18:21:14 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 3/4] mm, oom: do not rely on TIF_MEMDIE for
        exit_oom_victim

On 10/04, Michal Hocko wrote:
>
> -void release_task(struct task_struct *p)
> +bool release_task(struct task_struct *p)
>  {
>  	struct task_struct *leader;
>  	int zap_leader;
> +	bool last = false;
>  repeat:
>  	/* don't need to get the RCU readlock here - the process is dead and
>  	 * can't be modifying its own credentials. But shut RCU-lockdep up */
> @@ -197,8 +198,10 @@ void release_task(struct task_struct *p)
>  		 * then we are the one who should release the leader.
>  		 */
>  		zap_leader = do_notify_parent(leader, leader->exit_signal);
> -		if (zap_leader)
> +		if (zap_leader) {
>  			leader->exit_state = EXIT_DEAD;
> +			last = true;
> +		}
>  	}

This looks strange... it won't return true if "p" is the group leader.

> @@ -584,12 +587,15 @@ static void forget_original_parent(struct task_struct *father,
>  /*
>   * Send signals to all our closest relatives so that they know
>   * to properly mourn us..
> + *
> + * Returns true if this is the last thread from the thread group
>   */
> -static void exit_notify(struct task_struct *tsk, int group_dead)
> +static bool exit_notify(struct task_struct *tsk, int group_dead)
>  {
>  	bool autoreap;
>  	struct task_struct *p, *n;
>  	LIST_HEAD(dead);
> +	bool last = false;
>  
>  	write_lock_irq(&tasklist_lock);
>  	forget_original_parent(tsk, &dead);
> @@ -606,6 +612,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  	} else if (thread_group_leader(tsk)) {
>  		autoreap = thread_group_empty(tsk) &&
>  			do_notify_parent(tsk, tsk->exit_signal);
> +		last = thread_group_empty(tsk);

so this can't detect the multi-threaded group exit, and ...

>  	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
>  		list_del_init(&p->ptrace_entry);
> -		release_task(p);
> +		if (release_task(p) && p == tsk)
> +			last = true;

this can only happen if this process auto-reaps itself. Not to mention
that exit_notify() will never return true if traced.

No, this doesn't look right.

Oleg.