linux-kernel - Re: [PATCH -mm] scheduler: fix the return of the first time

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Date:	Mon, 16 Apr 2007 14:45:59 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
Cc:	Con Kolivas <kernel@...ivas.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH -mm] scheduler: fix the return of the first time_slice

On 04/16, Satoru Takeuchi wrote:
>
> --- linux-2.6.21-rc6-mm1.tsfix.orig/kernel/exit.c	2007-04-15 16:56:12.000000000 +0900
> +++ linux-2.6.21-rc6-mm1.tsfix/kernel/exit.c	2007-04-15 16:56:17.000000000 +0900
> @@ -680,10 +680,14 @@
>  			reaper = child_reaper(father);
>  			break;
>  		}
> +		if (reaper->time_slice_reaper == father)
> +			reaper->time_slice_reaper = NULL;
>  	} while (reaper->exit_state);
>  
>  	list_for_each_safe(_p, _n, &father->children) {
>  		p = list_entry(_p, struct task_struct, sibling);
> +		if (p->time_slice_reaper == father)
> +			p->time_slice_reaper = NULL;
>  		choose_new_parent(p, reaper);
>  		reparent_thread(p, father);
>  	}
> Index: linux-2.6.21-rc6-mm1.tsfix/kernel/sched.c
> ===================================================================
> --- linux-2.6.21-rc6-mm1.tsfix.orig/kernel/sched.c	2007-04-15 16:56:12.000000000 +0900
> +++ linux-2.6.21-rc6-mm1.tsfix/kernel/sched.c	2007-04-15 16:56:17.000000000 +0900
> @@ -1693,7 +1693,7 @@
>  	 * The remainder of the first timeslice might be recovered by
>  	 * the parent if the child exits early enough.
>  	 */
> -	p->first_time_slice = 1;
> +	p->time_slice_reaper = current;
>  	p->timestamp = sched_clock();
>  	local_irq_enable();

I am afraid this doesn't work for CLONE_THREAD. Suppose that some sub-thread (not
main thread) T1 creates another sub-thread, T2.

	T2->time_slice_reaper = T1.

then T1 exits, but forget_original_parent(T1) doesn't change T2->time_slice_reaper,
because T2 is not on T1->children list. Now T2->time_slice_reaper points to an
already freed task_struct.

Oleg.

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