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

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

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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