nfsd/create race fixes, infrastructure

new helpers - insert_inode_locked() and insert_inode_locked4().
Hash new inode, making sure that there's no such inode in icache
already.  If there is and it does not end up unhashed (as would
happen if we have nfsd trying to resolve a bogus fhandle), fail.
Otherwise insert our inode into hash and succeed.

In either case have i_state set to new+locked; cleanup ends up
being simpler with such calling conventions.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/inode.c

@@ -1032,6 +1032,65 @@ struct inode *iget_locked(struct super_block *sb, unsigned long ino)
 
 EXPORT_SYMBOL(iget_locked);
 
+int insert_inode_locked(struct inode *inode)
+{
+	struct super_block *sb = inode->i_sb;
+	ino_t ino = inode->i_ino;
+	struct hlist_head *head = inode_hashtable + hash(sb, ino);
+	struct inode *old;
+
+	inode->i_state |= I_LOCK|I_NEW;
+	while (1) {
+		spin_lock(&inode_lock);
+		old = find_inode_fast(sb, head, ino);
+		if (likely(!old)) {
+			hlist_add_head(&inode->i_hash, head);
+			spin_unlock(&inode_lock);
+			return 0;
+		}
+		__iget(old);
+		spin_unlock(&inode_lock);
+		wait_on_inode(old);
+		if (unlikely(!hlist_unhashed(&old->i_hash))) {
+			iput(old);
+			return -EBUSY;
+		}
+		iput(old);
+	}
+}
+
+EXPORT_SYMBOL(insert_inode_locked);
+
+int insert_inode_locked4(struct inode *inode, unsigned long hashval,
+		int (*test)(struct inode *, void *), void *data)
+{
+	struct super_block *sb = inode->i_sb;
+	struct hlist_head *head = inode_hashtable + hash(sb, hashval);
+	struct inode *old;
+
+	inode->i_state |= I_LOCK|I_NEW;
+
+	while (1) {
+		spin_lock(&inode_lock);
+		old = find_inode(sb, head, test, data);
+		if (likely(!old)) {
+			hlist_add_head(&inode->i_hash, head);
+			spin_unlock(&inode_lock);
+			return 0;
+		}
+		__iget(old);
+		spin_unlock(&inode_lock);
+		wait_on_inode(old);
+		if (unlikely(!hlist_unhashed(&old->i_hash))) {
+			iput(old);
+			return -EBUSY;
+		}
+		iput(old);
+	}
+}
+
+EXPORT_SYMBOL(insert_inode_locked4);
+
 /**
  *	__insert_inode_hash - hash an inode
  *	@inode: unhashed inode

include/linux/fs.h

@@ -1902,6 +1902,8 @@ extern struct inode *ilookup(struct super_block *sb, unsigned long ino);
 
 extern struct inode * iget5_locked(struct super_block *, unsigned long, int (*test)(struct inode *, void *), int (*set)(struct inode *, void *), void *);
 extern struct inode * iget_locked(struct super_block *, unsigned long);
+extern int insert_inode_locked4(struct inode *, unsigned long, int (*test)(struct inode *, void *), void *);
+extern int insert_inode_locked(struct inode *);
 extern void unlock_new_inode(struct inode *);
 
 extern void __iget(struct inode * inode);