On 10/18, Kirill Tkhai wrote:
>
> 18.10.2014, 01:40, "Oleg Nesterov" <o...@redhat.com>:
> > ...
> > The
> > task_struct itself can't go away,
> > ...
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1158,7 +1158,13 @@ static void task_numa_compare(struct task_numa_env
> > *env,
> >
> > rcu_read_lock();
> > cur = ACCESS_ONCE(dst_rq->curr);
> > - if (cur->pid == 0) /* idle */
> > + /*
> > + * No need to move the exiting task, and this ensures that ->curr
> > + * wasn't reaped and thus get_task_struct() in task_numa_assign()
> > + * is safe; note that rcu_read_lock() can't protect from the final
> > + * put_task_struct() after the last schedule().
> > + */
> > + if (is_idle_task(cur) || (cur->flags & PF_EXITING))
> > cur = NULL;
> >
> > /*
>
> Oleg, I've looked once again, and now it's not good for me.
Ah. Thanks a lot Kirill for correcting me!
I was looking at this rcu_read_lock() and I didn't even try to think
what it can actually protect. Nothing.
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1165,7 +1165,21 @@ static void task_numa_compare(struct task_numa_env
> *env,
>^^
> rcu_read_lock();
> cur = ACCESS_ONCE(dst_rq->curr);
> - if (cur->pid == 0) /* idle */
> + /*
> + * No need to move the exiting task, and this ensures that ->curr
> + * wasn't reaped and thus get_task_struct() in task_numa_assign()
> + * is safe; note that rcu_read_lock() can't protect from the final
> + * put_task_struct() after the last schedule().
> + */
> + if (is_idle_task(cur) || (cur->flags & PF_EXITING))
> + cur = NULL;
> + /*
> + * Check once again to be sure curr is still on dst_rq. Even if
> + * it points on a new task, which is using the memory of freed
> + * cur, it's OK, because we've locked RCU before
> + * delayed_put_task_struct() callback is called to put its struct.
> + */
> + if (cur != ACCESS_ONCE(dst_rq->curr))
No, I don't think this can work. Let's look at the current code:
rcu_read_lock();
cur = ACCESS_ONCE(dst_rq->curr);
if (cur->pid == 0) /* idle */
And any dereference, even reading ->pid is not safe. This memory can be
freed, unmapped, reused, etc.
Looks like, task_numa_compare() needs to take dst_rq->lock and get the
refernce first.
Or, perhaps, we need to change the rules to ensure that any "task_struct *"
pointer is rcu-safe. Perhaps we have more similar problems... I'd like to
avoid this if possible.
Hmm. I'll try to think more.
Thanks!
Oleg.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
