configfs: Introduce configfs_dirent_lock

This patch introduces configfs_dirent_lock spinlock to protect configfs_dirent
traversals against linkage mutations (add/del/move). This will allow
configfs_detach_prep() to avoid locking i_mutexes.

Locking rules for configfs_dirent linkage mutations are the same plus the
requirement of taking configfs_dirent_lock. For configfs_dirent walking, one can
either take appropriate i_mutex as before, or take configfs_dirent_lock.

The spinlock could actually be a mutex, but the critical sections are either
O(1) or should not be too long (default groups walking in last patch).

ChangeLog:
  - Clarify the comment on configfs_dirent_lock usage
  - Move sd->s_element init before linking the new dirent
  - In lseek(), do not release configfs_dirent_lock before the dirent is
    relinked.

Signed-off-by: Louis Rilling <Louis.Rilling@kerlabs.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/configfs/inode.c b/fs/configfs/inode.c
index b9a1d810346d..4803ccc94480 100644
--- a/fs/configfs/inode.c
+++ b/fs/configfs/inode.c
@@ -247,7 +247,9 @@ void configfs_hash_and_remove(struct dentry * dir, const char * name)
                 if (!sd->s_element)
                         continue;
                 if (!strcmp(configfs_get_name(sd), name)) {
+                        spin_lock(&configfs_dirent_lock);
                         list_del_init(&sd->s_sibling);
+                        spin_unlock(&configfs_dirent_lock);
                         configfs_drop_dentry(sd, dir);
                         configfs_put(sd);
                         break;