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

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.
 

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,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt