new helper: d_find_alias_rcu()

similar to d_find_alias(inode), except that
	* the caller must be holding rcu_read_lock()
	* inode must not be freed until matching rcu_read_unlock()
	* result is *NOT* pinned and can only be dereferenced until
the matching rcu_read_unlock().

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

fs/dcache.c

@@ -1042,6 +1042,31 @@ struct dentry *d_find_alias(struct inode *inode)
 }
 EXPORT_SYMBOL(d_find_alias);
 
+/*
+ *  Caller MUST be holding rcu_read_lock() and be guaranteed
+ *  that inode won't get freed until rcu_read_unlock().
+ */
+struct dentry *d_find_alias_rcu(struct inode *inode)
+{
+	struct hlist_head *l = &inode->i_dentry;
+	struct dentry *de = NULL;
+
+	spin_lock(&inode->i_lock);
+	// ->i_dentry and ->i_rcu are colocated, but the latter won't be
+	// used without having I_FREEING set, which means no aliases left
+	if (likely(!(inode->i_state & I_FREEING) && !hlist_empty(l))) {
+		if (S_ISDIR(inode->i_mode)) {
+			de = hlist_entry(l->first, struct dentry, d_u.d_alias);
+		} else {
+			hlist_for_each_entry(de, l, d_u.d_alias)
+				if (!d_unhashed(de))
+					break;
+		}
+	}
+	spin_unlock(&inode->i_lock);
+	return de;
+}
+
 /*
  *	Try to kill dentries associated with this inode.
  * WARNING: you must own a reference to inode.

include/linux/dcache.h

@@ -262,6 +262,8 @@ extern void d_tmpfile(struct dentry *, struct inode *);
 extern struct dentry *d_find_alias(struct inode *);
 extern void d_prune_aliases(struct inode *);
 
+extern struct dentry *d_find_alias_rcu(struct inode *);
+
 /* test whether we have any submounts in a subdir tree */
 extern int path_has_submounts(const struct path *);