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SubjectRe: fork_idle && pid problems ?
Oleg Nesterov wrote:
> 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.

Ouch... Indeed.

>>> 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);
> +

Hm... Looks ok, but I'd suggest such patch instead:

--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1348,6 +1348,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
}
attach_pid(p, PIDTYPE_PID, pid);
nr_threads++;
+ } else {
+ p->pids[PIDTYPE_PID].pid = NULL;
+ p->pids[PIDTYPE_SID].pid = NULL;
+ p->pids[PIDTYPE_PGID].pid = NULL;
}

total_forks++;
it will cover cases, when we (if ever) call the copy_process from
other place. Oh, well...

> return task;
> }
>
>
>



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