Semaphores, Nachos

Tom Kelliher, CS42

Oct. 2, 1996


  1. Let waits which cause busy waits actually block the process:

    Associate a ``blocked'' queue with each semaphore.

    typedef struct semaphore {
       int   value;
       pcb   *head;
    } semaphore;

    Semaphore creation:

    semaphore *createsem(int value) {
       semaphore   *sem;
       sem = get_next_sem();
       sem->value = value;
       sem->head = NULL;
       return (sem);
    void wait(semaphore *sem) {        /* need mutex goo here */
       if (--sem->value < 0) {
          <update status of current process>
          insqu(sem->head->prev, current);
    void signal(semaphore *sem) {         /* mutex */
       pcb   *proc;
       if (++sem->value <= 0) {
          proc = remqu(sem->head->next);
          <update status of proc>
          ordinsqu(ready, proc);
          if (proc->prio > current->prio)

Write a semaphore solution to the producer/consumer problem when you have N buffers.


Semaphore Implementation

Class declaration:

class Semaphore {
    Semaphore(char* debugName, int initialValue);  // set initial value
    ~Semaphore();                // de-allocate semaphore
    char* getName() { return name;}       // debugging assist
    void P();   // these are the only operations on a semaphore
    void V();   // they are both *atomic*
    char* name;        // useful for debugging
    int value;         // semaphore value, always >= 0
    List *queue;       // threads waiting in P() for the value to be > 0

Class definition:

// Semaphore::Semaphore
//    Initialize a semaphore, so that it can be used for synchronization.
// "debugName" is an arbitrary name, useful for debugging.
// "initialValue" is the initial value of the semaphore.

Semaphore::Semaphore(char* debugName, int initialValue)
    name = debugName;
    value = initialValue;
    queue = new List;

// Semaphore::Semaphore
//    De-allocate semaphore, when no longer needed.  Assume no one
// is still waiting on the semaphore!

    delete queue;

// Semaphore::P
//    Wait until semaphore value > 0, then decrement.  Checking the
// value and decrementing must be done atomically, so we
// need to disable interrupts before checking the value.
// Note that Thread::Sleep assumes that interrupts are disabled
// when it is called.

    IntStatus oldLevel = interrupt->SetLevel(IntOff); // disable interrupts
    while (value == 0) {         // semaphore not available
        queue->Append((void *)currentThread);  // so go to sleep
    value--;               // semaphore available, 
                  // consume its value
    (void) interrupt->SetLevel(oldLevel); // re-enable interrupts

// Semaphore::V
//    Increment semaphore value, waking up a waiter if necessary.
// As with P(), this operation must be atomic, so we need to disable
// interrupts.  Scheduler::ReadyToRun() assumes that threads
// are disabled when it is called.

    Thread *thread;
    IntStatus oldLevel = interrupt->SetLevel(IntOff);

    thread = (Thread *)queue->Remove();
    if (thread != NULL)    // make thread ready, consuming the V immediately
    (void) interrupt->SetLevel(oldLevel);

Wading through the code:

  1. currentThread?

  2. Sleep()?

  3. Interrupt levels?

  4. Nachos switches (

  5. Etc...


// The following class defines a "lock".  A lock can be BUSY or FREE.
// There are only two operations allowed on a lock: 
// Acquire -- wait until the lock is FREE, then set it to BUSY
// Release -- set lock to be FREE, waking up a thread waiting
//    in Acquire if necessary
// In addition, by convention, only the thread that acquired the lock
// may release it.  As with semaphores, you can't read the lock value
// (because the value might change immediately after you read it).  

class Lock {
    Lock(char* debugName);       // initialize lock to be FREE
    ~Lock();            // deallocate lock
    char* getName() { return name; }   // debugging assist

    void Acquire(); // these are the only operations on a lock
    void Release(); // they are both *atomic*

    bool isHeldByCurrentThread();   // true if the current thread
               // holds this lock.  Useful for
               // checking in Release, and in
               // Condition variable ops below.

    char* name;            // for debugging
    // plus some other stuff you'll need to define


Condition Variables

// The following class defines a "condition variable".  A condition
// variable does not have a value, but threads may be queued, waiting
// on the variable.  These are only operations on a condition variable: 
// Wait() -- release the lock, relinquish the CPU until signaled, 
//    then re-acquire the lock
// Signal() -- wake up a thread, if there are any waiting on 
//    the condition
// Broadcast() -- wake up all threads waiting on the condition
// All operations on a condition variable must be made while
// the current thread has acquired a lock.  Indeed, all accesses
// to a given condition variable must be protected by the same lock.
// In other words, mutual exclusion must be enforced among threads calling
// the condition variable operations.
// In Nachos, condition variables are assumed to obey *Mesa*-style
// semantics.  When a Signal or Broadcast wakes up another thread,
// it simply puts the thread on the ready list, and it is the responsibility
// of the woken thread to re-acquire the lock (this re-acquire is
// taken care of within Wait()).  By contrast, some define condition
// variables according to *Hoare*-style semantics -- where the signalling
// thread gives up control over the lock and the CPU to the woken thread,
// which runs immediately and gives back control over the lock to the 
// signaller when the woken thread leaves the critical section.
// The consequence of using Mesa-style semantics is that some other thread
// can acquire the lock, and change data structures, before the woken
// thread gets a chance to run.

class Condition {
    Condition(char* debugName);     // initialize condition to 
               // "no one waiting"
    ~Condition();       // deallocate the condition
    char* getName() { return (name); }
    void Wait(Lock *conditionLock);    // these are the 3 operations on 
               // condition variables; releasing the 
               // lock and going to sleep are 
               // *atomic* in Wait()
    void Signal(Lock *conditionLock);   // conditionLock must be held by
    void Broadcast(Lock *conditionLock);// the currentThread for all of 
               // these operations

    char* name;
    // plus some other stuff you'll need to define


Thomas P. Kelliher
Mon Sep 30 15:09:05 EDT 1996
Tom Kelliher