ceph: don't try to handle hashed dentries in non-O_CREAT atomic_open

If ceph_atomic_open is handed a !d_in_lookup dentry, then that means
that it already passed d_revalidate so we *know* that it's negative (or
at least was very recently). Just return -ENOENT in that case.

This also addresses a subtle bug in dentry handling. Non-O_CREAT opens
call atomic_open with the parent's i_rwsem shared, but calling
d_splice_alias on a hashed dentry requires the exclusive lock.

If ceph_atomic_open receives a hashed, negative dentry on a non-O_CREAT
open, and another client were to race in and create the file before we
issue our OPEN, ceph_fill_trace could end up calling d_splice_alias on
the dentry with the new inode with insufficient locks.

Cc: stable@vger.kernel.org
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/file.c

@@ -462,6 +462,9 @@ int ceph_atomic_open(struct inode *dir, struct dentry *dentry,
 		err = ceph_security_init_secctx(dentry, mode, &as_ctx);
 		if (err < 0)
 			goto out_ctx;
+	} else if (!d_in_lookup(dentry)) {
+		/* If it's not being looked up, it's negative */
+		return -ENOENT;
 	}
 
 	/* do the open */