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

Date: Fri, 30 Sep 2005 07:28:36 +1000
From: Herbert Xu <herbert@gondor.apana.org.au>

On Thu, Sep 29, 2005 at 09:02:29AM -0700, Suzanne Wood wrote:

quoted

The exchange below suggests that it is equally important 
to have the rcu_dereference() in __in_dev_get(), so the 
idea of the only difference between in_dev_get and 
__in_dev_get being the refcnt may be accepted.

With __in_dev_get() it's the caller's responsibility to ensure
that RCU works correctly.  Therefore if any rcu_dereference is
needed it should be done by the caller.

This sounds reasonable to me.  Does everyone agree? 

Some callers of __in_dev_get() don't need rcu_dereference at all
because they're protected by the rtnl.

BTW, could you please move the rcu_dereference in in_dev_get()
into the if clause? The barrier is not needed when ip_ptr is
NULL.

The trouble with that may be that there are three events, the
dereference, the assignment, and the conditional test.  The
rcu_dereference() is meant to assure deferred destruction
throughout.

Thank you very much for your suggestions.