Re: [PATCH tip/core/urgent 1/3] rcu: add rcu_access_pointer and rcu_dereference_protected

[David Howells <dhowells@redhat.com>]

Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> This patch adds variants of rcu_dereference() that handle situations
> where the RCU-protected data structure cannot change, perhaps due to
> our holding the update-side lock, or where the RCU-protected pointer is
> only to be fetched, not dereferenced.  These are needed due to some
> performance concerns with using rcu_dereference() where it is not
> required, aside from the need for lockdep/sparse checking.
> 
> The new rcu_access_pointer() primitive is for the case where the pointer
> is be fetch and not dereferenced.  This primitive may be used without
> protection, RCU or otherwise, due to the fact that it uses ACCESS_ONCE().
> 
> The new rcu_dereference_protected() primitive is for the case where updates
> are prevented, for example, due to holding the update-side lock.  This
> primitive does neither ACCESS_ONCE() nor smp_read_barrier_depends(), so
> can only be used when updates are somehow prevented.
> 
> Suggested-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Acked-by: David Howells <dhowells@redhat.com>