Re: [PATCH] sys_getppid oopses on debug kernel (v2)

From: Oleg Nesterov
Date: Wed Aug 09 2006 - 08:28:58 EST

On 08/09, Kirill Korotaev wrote:
> >Why do we need to use ->group_leader? All threads should have the same
> >->real_parent.
> I'm not sure this is true for old LinuxThreads...

Yes, from the user-space pov it may be not true, but ->group_leader->real_parent
should be equal ->real_parent anyway.

> >Why do we need tasklist_lock? I think rcu_read_lock() is enough.
> >
> >In other words, do you see any problems with this code
> >
> > smlinkage long sys_getppid(void)
> > {
> > int pid;
> >
> > rcu_read_lock();
> > pid = rcu_dereference(current->real_parent)->tgid;
> > rcu_read_unlock();
> >
> > return pid;
> > }
> >
> >? Yes, we may read a stale value for ->real_parent, but the memory
> >can't be freed while we are under rcu_read_lock(). And in this case
> >the returned value is ok because the task could be reparented just
> >after return anyway.
> Your patch doesn't cure the problem.
> rcu_read_lock just disables preemtion and rcu_dereference
> introduces memory barrier. _None_ of this _prevents_
> another CPU from freeing old real_parent in parallel with your dereference.

How so? Note that release_task() doesn't call put_task_struct(), it does
call_rcu(&p->rcu, delayed_put_task_struct) instead. When delayed_put_task_struct()
is called, all CPUs must see the new value of ->real_parent (otherwise
RCU is just broken). If CPU sees the old value of ->real_parent, rcu_read_lock()
protects us from delayed_put_task_struct() on another CPU.

Ok, I think this is the same "classic" pattern as:

old = global_ptr;
global_ptr = new;

Do you agree?


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