dcache: Reduce the scope of i_lock in d_splice_alias

i_lock is only needed until __d_find_any_alias calls dget on the alias
dentry.  After that the reference to new ensures that dentry_kill and
d_delete will not remove the inode from the dentry, and remove the
dentry from the inode->d_entry list.

The inode i_lock came to be held over the the __d_move calls in
d_splice_alias through a series of introduction of locks with
increasing smaller scope.  First it was the dcache_lock, then
it was the dcache_inode_lock, and finally inode->i_lock.

Furthermore inode->i_lock is not held over any other calls
to d_move or __d_move so it can not provide any meaningful
rename protection.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -2718,7 +2718,7 @@ 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, inode->i_lock and rename_lock
+ * dentry->d_parent->d_inode->i_mutex, and rename_lock
  *
  * Note: If ever the locking in lock_rename() changes, then please
  * remember to update this too...
@@ -2744,7 +2744,6 @@ static int __d_unalias(struct inode *inode,
 	__d_move(alias, dentry, false);
 	ret = 0;
 out_err:
-	spin_unlock(&inode->i_lock);
 	if (m2)
 		mutex_unlock(m2);
 	if (m1)
@@ -2790,10 +2789,11 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 	if (S_ISDIR(inode->i_mode)) {
 		struct dentry *new = __d_find_any_alias(inode);
 		if (unlikely(new)) {
+			/* The reference to new ensures it remains an alias */
+			spin_unlock(&inode->i_lock);
 			write_seqlock(&rename_lock);
 			if (unlikely(d_ancestor(new, dentry))) {
 				write_sequnlock(&rename_lock);
-				spin_unlock(&inode->i_lock);
 				dput(new);
 				new = ERR_PTR(-ELOOP);
 				pr_warn_ratelimited(
@@ -2812,7 +2812,6 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 			} else {
 				__d_move(new, dentry, false);
 				write_sequnlock(&rename_lock);
-				spin_unlock(&inode->i_lock);
 				security_d_instantiate(new, inode);
 			}
 			iput(inode);