[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, 8 insertions, 7 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 83beb1e93b18..6d2089a1bce7 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -411,15 +411,16 @@ void fastcall put_files_struct(struct files_struct *files)
                 close_files(files);
                 /*
                  * Free the fd and fdset arrays if we expanded them.
+                 * If the fdtable was embedded, pass files for freeing
+                 * at the end of the RCU grace period. Otherwise,
+                 * you can free files immediately.
                  */
                 fdt = files_fdtable(files);
-                if (fdt->fd != &files->fd_array[0])
-                        free_fd_array(fdt->fd, fdt->max_fds);
-                if (fdt->max_fdset > __FD_SETSIZE) {
-                        free_fdset(fdt->open_fds, fdt->max_fdset);
-                        free_fdset(fdt->close_on_exec, fdt->max_fdset);
-                }
-                kmem_cache_free(files_cachep, files);
+                if (fdt == &files->fdtab)
+                        fdt->free_files = files;
+                else
+                        kmem_cache_free(files_cachep, files);
+                free_fdtable(fdt);
         }
 }