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

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:

quoted

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.

quoted

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