Arrangements and methods for invoking an upcall in a computer system

A computer-implemented method for executing an application system call that involves invoking a kernel thread from a system call stub in a user space of an operating system of the computer. There is included saving user space context data in a save state upon entering a kernel space of the operating system from the user space. There is further included modifying a return pointer in the save state to an address of a unblock handler call stub in user space instead of an address of the system call stub in the user space, thereby causing the kernel thread to return to the unblock call handler stub instead of returning to the system call stub when the kernel thread completes execution.