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

From: Paul E. McKenney
Date: Wed Apr 07 2010 - 13:13:59 EST

On Wed, Apr 07, 2010 at 06:00:27PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > 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().
> > ...
> > +#define rcu_access_pointer(p, c) \
> NAK. This shouldn't have the conditional parameter 'c'. Given that 'c' (by
> analogy to rcu_dereference_check()) is there to describe the conditions under
> which it's permitted to dereference the pointer, why is that relevant here?
> What is it you're proving?

In some cases, the value of 'c' will indeed be '1', and in those cases,
there needs to be a comment stating why it is OK, similar to those
required for smp_mb() and friends.

In other cases, there will be a reference counter or a "not yet fully
initialized" flag that can (and should) be tested.

Thanx, Paul
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