Messages in this thread Patch in this message | | | Date | Thu, 17 Apr 2008 19:36:44 +0400 | From | Oleg Nesterov <> | Subject | Re: fork_idle && pid problems ? |
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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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