On Thu, Sep 29, 2005 at 05:23:46PM -0700, Paul E. McKenney wrote:
>
> Is there any case where __in_dev_get() might be called without
> needing to be wrapped with rcu_dereference()? If so, then I
> agree (FWIW, given my meagre knowledge of Linux networking).
Yes. All paths that call __in_dev_get() under the rtnl do not
need rcu_dereference (or any RCU at all) since the rtnl prevents
any ip_ptr modification from occuring.
> However, rcu_dereference() only generates a memory barrier on DEC
> Alpha, so there is normally no penalty for using it in the NULL-pointer
> case. So, when using rcu_dereference() unconditionally simplifies
> the code, it may make sense to "just do it".
Here is what the code would look like:
rcu_read_lock();
in_dev = dev->ip_ptr;
if (in_dev) {
in_dev = rcu_dereference(in_dev);
atomic_inc(&in_dev->refcnt);
}
rcu_read_unlock();
return in_dev;
Cheers,
--
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