configfs: Protect configfs_dirent s_links list mutations

Symlinks to a config_item are listed under its configfs_dirent s_links, but the
list mutations are not protected by any common lock.

This patch uses the configfs_dirent_lock spinlock to add the necessary
protection.

Note: we should also protect the list_empty() test in configfs_detach_prep() but
1/ the lock should not be released immediately because nothing would prevent the
list from being filled after a successful list_empty() test, making the problem
tricky,
2/ this will be solved by the rmdir() vs rename() deadlock bugfix.

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

2 files changed, 9 insertions, 4 deletions

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 2619f485bc3d..a08e5c2f25e8 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -37,10 +37,11 @@
 DECLARE_RWSEM(configfs_rename_sem);
 /*
  * Protects mutations of configfs_dirent linkage together with proper i_mutex
+ * Also protects mutations of symlinks linkage to target configfs_dirent
  * Mutators of configfs_dirent linkage must *both* have the proper inode locked
  * and configfs_dirent_lock locked, in that order.
- * This allows one to safely traverse configfs_dirent trees without having to
- * lock inodes.
+ * This allows one to safely traverse configfs_dirent trees and symlinks without
+ * having to lock inodes.
  */
 DEFINE_SPINLOCK(configfs_dirent_lock);
 
diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 676c84c416da..faeb4417a10d 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -77,12 +77,15 @@ static int create_link(struct config_item *parent_item,
         sl = kmalloc(sizeof(struct configfs_symlink), GFP_KERNEL);
         if (sl) {
                 sl->sl_target = config_item_get(item);
-                /* FIXME: needs a lock, I'd bet */
+                spin_lock(&configfs_dirent_lock);
                 list_add(&sl->sl_list, &target_sd->s_links);
+                spin_unlock(&configfs_dirent_lock);
                 ret = configfs_create_link(sl, parent_item->ci_dentry,
                                            dentry);
                 if (ret) {
+                        spin_lock(&configfs_dirent_lock);
                         list_del_init(&sl->sl_list);
+                        spin_unlock(&configfs_dirent_lock);
                         config_item_put(item);
                         kfree(sl);
                 }
@@ -186,8 +189,9 @@ int configfs_unlink(struct inode *dir, struct dentry *dentry)
                 type->ct_item_ops->drop_link(parent_item,
                                                sl->sl_target);
 
-        /* FIXME: Needs lock */
+        spin_lock(&configfs_dirent_lock);
         list_del_init(&sl->sl_list);
+        spin_unlock(&configfs_dirent_lock);
 
         /* Put reference from create_link() */
         config_item_put(sl->sl_target);