Re: RCU condition checks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Apr 07, 2010 at 05:35:30PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > > Why is there a need for 'c'?
> > 
> > An example use is where rcu_access_pointer() is legal because we are
> > either initializing or cleaning up, so that no other CPU has access
> > to the pointer.  In these cases, you might do something like:
> > 
> > 	q = rcu_access_pointer(p->a, p->refcnt == 0);
> 
> I think the main problem I have with this is that the fact that p->refcnt
> should be 0 here is unrelated to the fact that we're wanting to look at the
> value of p->a.  I'd say that this should be two separate statements, for
> example:
> 
> 	ASSERT(p->refcnt == 0);
> 	q = rcu_access_pointer(p->a);
> 
> I could see using a lockdep-managed ASSERT here would work, though.
> 
> The other problem I have with this is that I'm assuming rcu_access_pointer()
> is simply for looking at the value of the pointer without dereferencing it -
> in which case, is there any need for the lock-describing condition?

I agree that in many cases there won't be a reasonable condition.
In which case, using "1" and an explanatory comment makes sense.
In other cases, the fact that the value is zero can mean that no one
else can possibly have a reference.

All that aside, I fully expect that uses of rcu_access_pointer() will
require more than the usual code-review effort, as these sorts of
unprotected accesses are notoriously error-prone.

> I agree, though, that:
> 
> 	q = rcu_dereference_check(p->a,
> 				  rcu_read_lock_held() || (
> 				   lockdep_is_held(p->lock) &&
> 				   lockdep_is_held(q->lock)));
> 
> is a reasonable way of keeping the dereference and the lock checks together,
> though that could equally well be written, say:
> 
> 	LOCKDEP_ASSERT(rcu_read_lock_held() || (
> 		        lockdep_is_held(p->lock) &&
> 			lockdep_is_held(q->lock)));
> 	q = rcu_dereference_protected(p->a);
> 
> but combining those makes it easier to ensure people to write lock checking.

Glad you like it!

							Thanx, Paul