Re: 3.9-rc1 NULL pointer crash at find_pid_ns

Eric Dumazet
2013-03-07T18:18:48Z

On Thu, 2013-03-07 at 13:14 -0500, Sasha Levin wrote:

> Okay, I'm even more confused now.
>
> The expression in question is:
>
> hlist_entry_safe(rcu_dereference_bh(hlist_first_rcu(head)))
>
> You're saying that "rcu_dereference_bh(hlist_first_rcu(head))" can change between
> the two evaluations we do. That would mean that 'head' has changed in between, right?
>
> In that case, the list itself has changed - which means that RCU has changed the
> list underneath us.
>
> hlist_first_rcu() doesn't have any side-effects, it doesn't modify the list whatsoever,
> so the only thing that can change is 'head'. Why is it allowed to change if the list
> is protected by RCU?

Thats the point of RCU ;)

Things can change under us.