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

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

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