[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>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 94aefa54a1b..68ab5f2ab9c 100644
--- a/include/linux/init_task.h
+++ b/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, } },                   \