fix loop checks in d_materialise_unique()

Both __d_unalias() and __d_materialise_dentry() need loop prevention.
Grab rename_lock in caller, check for loops there...

As a side benefit, we have dentry_lock_for_move() called only under
rename_lock, which seriously reduces deadlock potential of the
execrable "locking order" used for ->d_lock.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 34 insertions, 17 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 37f72ee5bf7c..6e4ea6d87774 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2213,14 +2213,15 @@ static void dentry_unlock_parents_for_move(struct dentry *dentry,
  * The hash value has to match the hash queue that the dentry is on..
  */
 /*
- * d_move - move a dentry
+ * __d_move - move a dentry
  * @dentry: entry to move
  * @target: new dentry
  *
  * Update the dcache to reflect the move of a file name. Negative
- * dcache entries should not be moved in this way.
+ * dcache entries should not be moved in this way.  Caller hold
+ * rename_lock.
  */
-void d_move(struct dentry * dentry, struct dentry * target)
+static void __d_move(struct dentry * dentry, struct dentry * target)
 {
         if (!dentry->d_inode)
                 printk(KERN_WARNING "VFS: moving negative dcache entry\n");
@@ -2228,8 +2229,6 @@ void d_move(struct dentry * dentry, struct dentry * target)
         BUG_ON(d_ancestor(dentry, target));
         BUG_ON(d_ancestor(target, dentry));
 
-        write_seqlock(&rename_lock);
-
         dentry_lock_for_move(dentry, target);
 
         write_seqcount_begin(&dentry->d_seq);
@@ -2275,6 +2274,20 @@ void d_move(struct dentry * dentry, struct dentry * target)
         spin_unlock(&target->d_lock);
         fsnotify_d_move(dentry);
         spin_unlock(&dentry->d_lock);
+}
+
+/*
+ * d_move - move a dentry
+ * @dentry: entry to move
+ * @target: new dentry
+ *
+ * Update the dcache to reflect the move of a file name. Negative
+ * dcache entries should not be moved in this way.
+ */
+void d_move(struct dentry *dentry, struct dentry *target)
+{
+        write_seqlock(&rename_lock);
+        __d_move(dentry, target);
         write_sequnlock(&rename_lock);
 }
 EXPORT_SYMBOL(d_move);
@@ -2302,7 +2315,7 @@ struct dentry *d_ancestor(struct dentry *p1, struct dentry *p2)
  * This helper attempts to cope with remotely renamed directories
  *
  * It assumes that the caller is already holding
- * dentry->d_parent->d_inode->i_mutex and the inode->i_lock
+ * dentry->d_parent->d_inode->i_mutex, inode->i_lock and rename_lock
  *
  * Note: If ever the locking in lock_rename() changes, then please
  * remember to update this too...
@@ -2317,11 +2330,6 @@ static struct dentry *__d_unalias(struct inode *inode,
         if (alias->d_parent == dentry->d_parent)
                 goto out_unalias;
 
-        /* Check for loops */
-        ret = ERR_PTR(-ELOOP);
-        if (d_ancestor(alias, dentry))
-                goto out_err;
-
         /* See lock_rename() */
         ret = ERR_PTR(-EBUSY);
         if (!mutex_trylock(&dentry->d_sb->s_vfs_rename_mutex))
@@ -2331,7 +2339,7 @@ static struct dentry *__d_unalias(struct inode *inode,
                 goto out_err;
         m2 = &alias->d_parent->d_inode->i_mutex;
 out_unalias:
-        d_move(alias, dentry);
+        __d_move(alias, dentry);
         ret = alias;
 out_err:
         spin_unlock(&inode->i_lock);
@@ -2416,15 +2424,24 @@ struct dentry *d_materialise_unique(struct dentry *dentry, struct inode *inode)
                 alias = __d_find_alias(inode, 0);
                 if (alias) {
                         actual = alias;
-                        /* Is this an anonymous mountpoint that we could splice
-                         * into our tree? */
-                        if (IS_ROOT(alias)) {
+                        write_seqlock(&rename_lock);
+
+                        if (d_ancestor(alias, dentry)) {
+                                /* Check for loops */
+                                actual = ERR_PTR(-ELOOP);
+                        } else if (IS_ROOT(alias)) {
+                                /* Is this an anonymous mountpoint that we
+                                 * could splice into our tree? */
                                 __d_materialise_dentry(dentry, alias);
+                                write_sequnlock(&rename_lock);
                                 __d_drop(alias);
                                 goto found;
+                        } else {
+                                /* Nope, but we must(!) avoid directory
+                                 * aliasing */
+                                actual = __d_unalias(inode, dentry, alias);
                         }
-                        /* Nope, but we must(!) avoid directory aliasing */
-                        actual = __d_unalias(inode, dentry, alias);
+                        write_sequnlock(&rename_lock);
                         if (IS_ERR(actual))
                                 dput(alias);
                         goto out_nolock;