+	if (txwin->user_win) {
+		/*
+		 * Window opened by child thread may not be closed when
+		 * it exits. So take reference to its pid and release it
+		 * when the window is free by parent thread.
+		 * Acquire a reference to the task's pid to make sure
+		 * pid will not be re-used.
+		 */
+		txwin->pid = get_task_pid(current, PIDTYPE_PID);
+		/*
+		 * Acquire a reference to the task's mm.
+		 */
+		txwin->mm = get_task_mm(current);
+
+		if (txwin->mm) {
+			mmput(txwin->mm);
+			mmgrab(txwin->mm);

Doesn't the mmgrab need to come before the mmput?

+			mm_context_add_copro(txwin->mm);
+		} else {
+			put_pid(txwin->pid);
+			pr_err("VAS: pid(%d): mm_struct is not found\n",
+					current->pid);
+			rc = -EPERM;
+			goto free_window;
+		}

Also the code is much easier to follow if you handle the error
first and avoid the else:

		txwin->mm = get_task_mm(current);
		if (!txwin->mm) {
			put_pid(txwin->pid);
			pr_err("VAS: pid(%d): mm_struct is not found\n",
					current->pid);
			rc = -EPERM;
			goto free_window;
		}
		mmgrab(txwin->mm);
		mmput(txwin->mm);

Also don't you need to take a reference to the struct pid for the
tgid as well?