Goal of this patch is to not touch inode_lock for socket/pipes/anonfd
 inodes allocation/freeing.

 In new_inode(), we test if super block has MS_SPECIAL flag set.
 If yes, we dont put inode in "inode_in_use" list nor "sb->s_inodes" list
 As inode_lock was taken only to protect these lists, we avoid it as well

 Using iput_special() from dput_special() avoids taking inode_lock
 at freeing time.

 This patch has a very noticeable effect, because we avoid dirtying 
 of three contended cache lines in new_inode(), and five cache lines
 in iput()

Note: Not sure if we can use MS_SPECIAL=MS_NOUSER, or if we
really need a different flag.

(socket8 bench result : from 20.5s to 2.94s) 

Signed-off-by: Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/YWAg@public.gmane.org>
---

 fs/anon_inodes.c   |    1 +
 fs/dcache.c        |    2 +-
 fs/inode.c         |   25 ++++++++++++++++++-------
 fs/pipe.c          |    3 ++-
 include/linux/fs.h |    2 ++
 net/socket.c       |    1 +
 6 files changed, 25 insertions(+), 9 deletions(-)