From: Maneesh Soni <maneesh@in.ibm.com>

While path walking we do follow_mount or follow_down which uses
dcache_lock for serialisation.  vfsmount related operations also use
dcache_lock for all updates. I think we can use a separate lock for
vfsmount related work and can improve path walking.

The following two patches does the same. The first one replaces
dcache_lock with new vfsmount_lock in namespace.c. The lock is
local to namespace.c and is not required outside. The second patch
uses RCU to have lock free lookup_mnt(). The patches are quite simple
and straight forward.

The lockmeter reults show reduced contention, and lock acquisitions
for dcache_lock while running dcachebench* on a 4-way SMP box

    SPINLOCKS         HOLD            WAIT
    UTIL  CON    MEAN(  MAX )   MEAN(  MAX )(% CPU)     TOTAL NOWAIT SPIN RJECT  NAME

  baselkm-2569:
    20.7% 20.9%  0.5us( 146us)  2.9us( 144us)(0.81%)  31590840 79.1% 20.9%    0%  dcache_lock
  mntlkm-2569:
    14.3% 13.6%  0.4us( 170us)  2.9us( 187us)(0.42%)  23071746 86.4% 13.6%    0%  dcache_lock

We get more than 8% improvement on 4-way SMP and 44% improvement on 16-way
NUMAQ while runing dcachebench*.

		Average (usecs/iteration)	Std. Deviation
		(lower is better)
4-way SMP
  2.5.69	15739.3				470.90
  2.5.69-mnt	14459.6				298.51

16-way NUMAQ
  2.5.69	120426.5			363.78
  2.5.69-mnt	 63225.8			427.60

*dcachebench is a microbenchmark written by Bill Hartner and is available at
http://www-124.ibm.com/developerworks/opensource/linuxperf/dcachebench/dcachebench.html

 vfsmount_lock.patch
 -------------------
 - Patch for replacing dcache_lock with new vfsmount_lock for all mount
   related operation. This removes the need to take dcache_lock while
   doing follow_mount or follow_down operations in path walking.

I re-ran dcachebench with 2.5.70 as base on 16-way NUMAQ box.

                	Average (usecs/iteration)       Std. Deviation
                	(lower is better)
16-way NUMAQ
2.5.70 				120710.9		 	230.67
 + vfsmount_lock.patch  	65209.6				242.97
    + lookup_mnt-rcu.patch 	64042.3				416.61

So just the lock splitting (vfsmount_lock.patch) gives almost similar benifits