Hi all

Here is a serie of three patches (based on net-next-2.6), to continue work
with RCU on UDP/TCP/DCCP stacks

Many thanks for all usefull reviews and comments, especially from Paul and Corey.

1) Introduce hlist_nulls variant of hlist

   hlist uses NULL value to finish a chain.
   hlist_nulls variant use the low order bit set to 1 to signal an end marker.
   This allows to store many different end markers, so that some RCU lockless
   algos (used in TCP/UDP stack for example) can save some memory barriers in
   fast paths.

2) Use hlist_nulls in UDP RCU code

   This is a straightforward patch, using hlist_nulls infrastructure.
   RCU-ification already done on UDP two weeks ago, so hlist_nulls
   permits us to avoid some memory barriers, both at lookup time
   and delete time. Patch is large because it adds new macros to
   include/net/sock.h. These macros will be used by TCP & DCCP too.

3) Convert TCP & DCCP hash tables to use RCU & hlist_nulls

   RCU was added to UDP lookups, using a fast infrastructure :
   - sockets kmem_cache use SLAB_DESTROY_BY_RCU and dont pay the
     price of call_rcu() at freeing time.
   - hlist_nulls permits to use few memory barriers.

   This patch uses same infrastructure for TCP/DCCP established
   and timewait sockets.

   Thanks to SLAB_DESTROY_BY_RCU, no slowdown for applications
   using short lived TCP connections. A followup patch, converting
   rwlocks to spinlocks will even speedup this case.

   __inet_lookup_established() is pretty fast now we dont have to
   dirty a contended cache line (read_lock/read_unlock)

   Only established and timewait hashtable are converted to RCU
  (bind table and listen table are still using traditional locking)


Signed-off-by: Eric Dumazet <redacted>