This patch uses the rcu_assign_pointer() API to eliminate a number of explicit
memory barriers from the SysV IPC code that uses RCU.  It also restructures
the ipc_ids structure so that the array size is stored in the same memory
block as the array itself (see the new struct ipc_id_ary).  This prevents the
race that the earlier code was subject to, where a reader could see a mismatch
between the size and the actual array.  With the size stored with the array,
the possibility of mismatch is eliminated -- with out the need for careful
ordering and explicit memory barriers.  This has been tested successfully on
i386 and ppc64.

Signed-off-by: <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>