Locks and Condition Variables

Lecture Notes for CS 140
Spring 2020
John Ousterhout

• Readings for this topic from Operating Systems: Principles and Practice: Sections 5.2-5.4.

• Needed: higher-level synchronization mechanism that provides
  • Mutual exclusion: easy to create critical sections
  • Blocking: delay a thread until some desired event occurs

Locks

• Lock: an object that can only be owned by a single thread at any given time. Basic operations on a lock:
  • acquire: mark the lock as owned by the current thread; if some other thread already owns the lock then first wait until the lock is free. Lock typically includes a queue to keep track of multiple waiting threads.
  • release: mark the lock as free (it must currently be owned by the calling thread).

• Too much milk solution with locks (using Pintos APIs):

  struct lock l;
  ...
  lock_acquire(&l);
  if (milk == 0) {
    buy_milk();
  }
  lock_release(&l);
  
• A more complex example: producer/consumer.
  • Producers add characters to a buffer
  • Consumers remove characters from the buffer
  • Characters will be removed in the same order added
  • Version 1:

    char buffer[SIZE];
    int count = 0, putIndex = 0, getIndex = 0;
    struct lock l;
    lock_init(&l);
    
    void put(char c) {
        lock_acquire(&l);
        count++;
        buffer[putIndex] = c;
        putIndex++;
        if (putIndex == SIZE) {
            putIndex = 0;
        }
        lock_release(&l);
    }
    
    char get() {
        char c;
        lock_acquire(&l);
        count--;
        c = buffer[getIndex];
        getIndex++;
        if (getIndex == SIZE) {
            getIndex = 0;
        }
        lock_release(&l);
        return c;
    }
    

  • Version 2 (handle empty/full cases):

    char buffer[SIZE];
    int count = 0, putIndex = 0, getIndex = 0;
    struct lock l;
    lock_init(&l);
    
    void put(char c) {
        lock_acquire(&l);
        while (count == SIZE) {
            lock_release(&l);
            lock_acquire(&l);
        }
        count++;
        buffer[putIndex] = c;
        putIndex++;
        if (putIndex == SIZE) {
            putIndex = 0;
        }
        lock_release(&l);
    }
    
    char get() {
        char c;
        lock_acquire(&l);
        while (count == 0) {
            lock_release(&l);
            lock_acquire(&l);
        }
        count--;
        c = buffer[getIndex];
        getIndex++;
        if (getIndex == SIZE) {
            getIndex = 0;
        }
        lock_release(&l);
        return c;
    }
    

Condition Variables

• Synchronization mechanisms need more than just mutual exclusion; also need a way to wait for another thread to do something (e.g., wait for a character to be added to the buffer)
• Condition variables: used to wait for a particular state to be reached (e.g. characters in buffer).
  • wait(condition, lock): atomically release lock, put thread to sleep until condition is signaled; when thread wakes up again, re-acquire lock before returning.
  • signal(condition, lock): if any threads are waiting on condition, wake up one of them. Caller must hold lock, which must be the same as the lock used in the wait call.
  • broadcast(condition, lock): same as signal, except wake up all waiting threads.
  • Note: after signal, signaling thread keeps lock, waking thread goes on the queue waiting for the lock.
  • Warning: when a thread wakes up after cond_wait there is no guarantee that the desired condition still exists: another thread might have snuck in.
• Producer/Consumer, version 3 (with condition variables):

  char buffer[SIZE];
  int count = 0, putIndex = 0, getIndex = 0;
  struct lock l;
  struct condition charAdded;
  struct condition charRemoved;
  
  lock_init(&l);
  cond_init(&charAdded);
  cond_init(&charRemoved);
  
  void put(char c) {
      lock_acquire(&l);
      while (count == SIZE) {
          cond_wait(&charRemoved, &l);
      }
      count++;
      buffer[putIndex] = c;
      putIndex++;
      if (putIndex == SIZE) {
          putIndex = 0;
      }
      cond_signal(&charAdded, &l);
      lock_release(&l);
  }
  
  char get() {
      char c;
      lock_acquire(&l);
      while (count == 0) {
          cond_wait(&charAdded, &l);
      }
      count--;
      c = buffer[getIndex];
      getIndex++;
      if (getIndex == SIZE) {
          getIndex = 0;
      }
      cond_signal(&charRemoved, &l);
      lock_release(&l);
      return c;
  }
  

Monitors

• When locks and condition variables are used together like this, the result is called a monitor:
  • A shared data structure
  • A collection of procedures
  • One lock that must be held whenever accessing the shared data (typically each procedure acquires the lock at the very beginning and releases the lock before returning).
  • One or more condition variables used for waiting.

• There are other synchronization mechanisms besides locks and condition variables. Be sure to read about semaphores in the book or in the Pintos documentation.