Re: [RFC][PATCH] identify in_dev_get rcu read-side critical sections

[Paul E. McKenney]

On Fri, Sep 30, 2005 at 11:04:04AM +1000, Herbert Xu wrote:
> On Thu, Sep 29, 2005 at 05:36:42PM -0700, Paul E. McKenney wrote:
> >
> > > 	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;
> 
> With this the barrier will taken even when in_dev is NULL.
> 
> I agree this isn't such a big deal since it only impacts Alpha and then
> only when in_dev is NULL.  But as we already do the branch anyway to
> increment the reference count, we might as well make things a little
> better for Alpha.

OK, how about this instead?

	rcu_read_lock();
	in_dev = dev->ip_ptr;
	if (in_dev) {
		atomic_inc(&rcu_dereference(in_dev)->refcnt);
	}
	rcu_read_unlock();
	return in_dev;

						Thanx, Paul
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