linux-kernel - Re: fork_idle && pid problems ?

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Message-ID: <20080417153644.GA69@tv-sign.ru>
Date:	Thu, 17 Apr 2008 19:36:44 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Pavel Emelyanov <xemul@...nvz.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Jesper Juhl <jesper.juhl@...il.com>,
	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>
Subject: Re: fork_idle && pid problems ?

On 04/17, Pavel Emelyanov wrote:
>
> Oleg Nesterov wrote:
> > 
> > But wait... What _is_ the task_pid() after fork_idle() ???
> 
> It is NULL, but every code getting one can handle such case :)
> 
> > fork_idle() doesn't really attach the new thread to the init_struct_pid,
> > so ->pids[PIDTYPE_PID].pid just points the parent's pid, no?
> > 
> > As for x86, the parent is /sbin/init (kernel_init->smp_prepare_cpus),
> > not so bad, it can't exit.
> > 
> > But what about HOTPLUG_CPU? Suppose we add CPU, use some non-idle
> > kernel thread (workqueue) to fork the idle thread. CPU goes down,
> > parent exits and frees the pid. Now, if this CPU goes up again, the
> > idle thread runs with its ->pid pointing to the freed memory, not
> > good.
> 
> Nope - it will be NULL.

How so? I bet it won't be NULL...

	dup_task_struct:

		*tsk = *orig;

After that the child's ->pids[PIDTYPE_MAX] is a copy of parent's.
But the task is not attached to these pids.

> > Not serious perhaps, afaics we only need this ->pid to ensure that
> > swapper can safely fork /sbin/init, but still.
> > 
> > Pavel, Eric, Sukadev? Please say I missed something! ;)
> > 
> > Otherwise, we can change init_idle() to do attach_pid(init_struct_pid),
> > afaics we can do this lockless. In that case we should also change
> > INIT_STRUCT_PID() and remove the initialization of .tasks.
> 
> Well, these was some request to make tasks always have pid link
> point to not NULL (from Matt?) so we'll need this :)

For now I'd suggest the patch below. If contrary to our expectations
there is any usage of idle_task->pids, we will notice ;)

Oleg.

--- kernel/fork.c~	2008-03-07 18:11:27.000000000 +0300
+++ kernel/fork.c	2008-04-17 19:34:10.000000000 +0400
@@ -1420,6 +1420,9 @@ struct task_struct * __cpuinit fork_idle
 	if (!IS_ERR(task))
 		init_idle(task, cpu);
 
+	/* COMMENT */
+	memset(task->pids, 0, sizeof task->pids);
+
 	return task;
 }
 

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