On Fri, Sep 30, 2005 at 10:27:19AM +1000, Herbert Xu wrote:
> 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;
How about:
rcu_read_lock();
in_dev = dev->ip_ptr;
if (rcu_dereference(in_dev)) {
atomic_inc(&in_dev->refcnt);
}
rcu_read_unlock();
return in_dev;
Admittedly only saves one line, but...
Thanx, Paul
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