[PATCH] files: files struct with RCU

Patch to eliminate struct files_struct.file_lock spinlock on the reader side
and use rcu refcounting rcuref_xxx api for the f_count refcounter.  The
updates to the fdtable are done by allocating a new fdtable structure and
setting files->fdt to point to the new structure.  The fdtable structure is
protected by RCU thereby allowing lock-free lookup.  For fd arrays/sets that
are vmalloced, we use keventd to free them since RCU callbacks can't sleep.  A
global list of fdtable to be freed is not scalable, so we use a per-cpu list.
If keventd is already handling the current cpu's work, we use a timer to defer
queueing of that work.

Since the last publication, this patch has been re-written to avoid using
explicit memory barriers and use rcu_assign_pointer(), rcu_dereference()
premitives instead.  This required that the fd information is kept in a
separate structure (fdtable) and updated atomically.

Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/init_task.h

@@ -2,6 +2,7 @@
 #define _LINUX__INIT_TASK_H
 
 #include <linux/file.h>
+#include <linux/rcupdate.h>
 
 #define INIT_FDTABLE \
 {							\
@@ -11,12 +12,16 @@
 	.fd		= &init_files.fd_array[0], 	\
 	.close_on_exec	= &init_files.close_on_exec_init, \
 	.open_fds	= &init_files.open_fds_init, 	\
+	.rcu		= RCU_HEAD_INIT, 		\
+	.free_files	= NULL,		 		\
+	.next		= NULL,		 		\
 }
 
 #define INIT_FILES \
 { 							\
 	.count		= ATOMIC_INIT(1), 		\
 	.file_lock	= SPIN_LOCK_UNLOCKED, 		\
+	.fdt		= &init_files.fdtab, 		\
 	.fdtab		= INIT_FDTABLE,			\
 	.close_on_exec_init = { { 0, } }, 		\
 	.open_fds_init	= { { 0, } }, 			\