On Fri, Nov 30, 2007 at 10:49:00AM +1100, Herbert Xu wrote:

On Thu, Nov 29, 2007 at 06:36:50AM -0800, Paul E. McKenney wrote:

quoted

That certainly is an interesting tradeoff...  Save a memory barrier
when assigning NULL, but pay an extra test and branch in all cases.
Though it does make for a simpler rule -- just use rcu_assign_pointer()
in all cases.  Of course, if almost all rcu_assign_pointer() executions
assign non-NULL pointers, the optimal strategy would be to leave the
implementation of rcu_assign_pointer() alone, and simply enforce use
of rcu_assign_pointer(), even if the pointer being assigned is NULL.

I was thinking of something much simpler.  If the second argument is
constant and NULL, then skip the barrier.  No run-time slow-down at
all.

That certainly makes a lot of sense!!!

You have in mind something like the following?

#define rcu_assign_pointer(p, v) \
	({ \
		if (!__builtin_constant_p(v) || \
		    ((v) != NULL)) \
			smp_wmb(); \
		(p) = (v); \
	})

If so, I will do some testing and submit a patch.  Probably to Gautham's
preemptible-RCU patchset to avoid gratuitously complicating his life,
especially given that he very graciously agreed to take it over from me.
We should be able to live with the overhead in the meantime.  ;-)

							Thanx, Paul

quoted

Although rcu_dereference() does a memory barrier only on Alpha, that of
rcu_assign_pointer() is needed on any machine that does not preserve store
order (Itanium, POWER, ARM, some MIPS boxes according to rumor, ...).

Good point!

Thanks,
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