Homework 6

Sarah Diesburg

Operating Systems

CS 3430

Helicopter Problem

A helicopter ride has five seats, and it always carries a full load.

Use lock(s) and condition variable(s) to write a procedure PersonArrives(), which is called whenever a person (thread) arrives.

Once the load is full, one person (thread) should call UpAndAway(), and five threads should return from PersonArrives(). There should be no undue waiting: the helicopter

should depart as soon as it has a full load.

Helicopter Problem

int queue = 0;condition okToGo = NULL;Lock lock = FREE;

PersonArrives() {lock.Acquire();++queue;if (queue < 4) {

okToGo.wait(&lock);} else {

GoGoGo(); queue = 0;

okToGo.Signal(&lock);okToGo.Signal(&lock);okToGo.Signal(&lock);

}lock.Release();

}

Helicopter Problem

int queue = 0;condition okToGo = NULL;Lock lock = FREE;

PersonArrives() {lock.Acquire();++queue;if (queue < 5) {

okToGo.wait(&lock);} else {

UpAndAway();= 0; okToGo.Signal(&lock);okToGo.Signal(&lock);okToGo.Signal(&lock);okToGo.Signal(&lock);

}lock.Release();

}

Helicopter Problem

int queue = 0;condition okToGo = NULL;Lock lock = FREE;

PersonArrives() {lock.Acquire();++queue;if (queue < 5) {

okToGo.wait(&lock);} else {

UpAndAway(); okToGo.Signal(&lock);

okToGo.Signal(&lock);okToGo.Signal(&lock);okToGo.Signal(&lock);queue = 0;

}lock.Release();

}

Proving Correctness

Mutual exclusion All shared variables are locked

e.g., queue Signal() and Wait() are invoked between

lock.Acquire() and lock.Release()

Proving Correctness

Liveness Maximum of 4 threads in okToGo.wait() Every 5th thread wakes up exactly four threads

already waiting in okToGo.wait() Additional threads wait at lock.Acquire() As long as we have 5n threads, every thread will

make progress

Proving Correctness

Fairness As long as we have FIFO queues at

lock.Acquire() The queuing policy at okToGo.wait() does not

matter, since all four waiting threads will be awakened

As long as we have 5n threads Every thread will get its chance

Common Pitfalls

int queue = 0;condition okToGo = NULL;Lock lock = FREE;

PersonArrives() {lock.Acquire();++queue;if (queue < 5) {

okToGo.wait(&lock);} else {

UpAndAway(); okToGo.Signal(&lock);

okToGo.Signal(&lock);okToGo.Signal(&lock); okToGo.Signal(&lock);queue = 0;lock.Release();

}}

Wait() requires lock on returnNo lock.Release()

Common Pitfalls

int queue = 0;condition okToGo = NULL;Lock lock = FREE;

PersonArrives() {lock.Acquire();++queue;lock.Release();if (queue < 5) {

lock.Acquire();okToGo.wait(&lock);lock.Release();

} else { lock.Acquire();UpAndAway();

okToGo.Signal(&lock);okToGo.Signal(&lock);okToGo.Signal(&lock);okToGo.Signal(&lock); queue = 0; lock.Release();

}}

Unprotected states

Multiple threads may wait here

Kernel Modules

A kernel module is driver code that is dynamically loaded into the driver when it is needed Also can be unloaded

The kernel is an executable file Can contain built-in driver code, but makes the

executable bigger

Module Commands

insmod – insert a module into the running kernel

rmmod – remove a module from the running kernel

modprobe – inserts a module, along with all other modules it depends on Only using insmod for our project