[PATCH] list: add missing rcu_dereference on first element

It seems that all the list_*_rcu primitives are missing a memory barrier
on the very first dereference.  For example,

#define list_for_each_rcu(pos, head) \
	for (pos = (head)->next; prefetch(pos->next), pos != (head); \
		pos = rcu_dereference(pos->next))

It will go something like:

	pos = (head)->next

	prefetch(pos->next)

	pos != (head)

	do stuff

We're missing a barrier here.

	pos = rcu_dereference(pos->next)

		fetch pos->next

		barrier given by rcu_dereference(pos->next)

		store pos

Without the missing barrier, the pos->next value may turn out to be stale.
In fact, if "do stuff" were also dereferencing pos and relying on
list_for_each_rcu to provide the barrier then it may also break.

So here is a patch to make sure that we have a barrier for the first
element in the list.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>