Re: [PATCH tip/urgent] rcu: add rcu_access_pointer andrcu_dereference_protected

From: Eric Dumazet
Date: Wed Apr 07 2010 - 12:39:07 EST

Le mercredi 07 avril 2010 Ã 09:26 -0700, Paul E. McKenney a Ãcrit :
> 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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Acked-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>

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