Initial version

code/threads/synch.h

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+// synch.h 
+//      Data structures for synchronizing threads.
+//
+//      Three kinds of synchronization are defined here: semaphores,
+//      locks, and condition variables.  The implementation for
+//      semaphores is given; for the latter two, only the procedure
+//      interface is given -- they are to be implemented as part of 
+//      the first assignment.
+//
+//      Note that all the synchronization objects take a "name" as
+//      part of the initialization.  This is solely for debugging purposes.
+//
+// Copyright (c) 1992-1993 The Regents of the University of California.
+// All rights reserved.  See copyright.h for copyright notice and limitation 
+// synch.h -- synchronization primitives.  
+
+#ifndef SYNCH_H
+#define SYNCH_H
+
+#include "copyright.h"
+#include "thread.h"
+#include "list.h"
+
+// The following class defines a "semaphore" whose value is a non-negative
+// integer.  The semaphore has only two operations P() and V():
+//
+//      P() -- waits until value > 0, then decrement
+//
+//      V() -- increment, waking up a thread waiting in P() if necessary
+// 
+// Note that the interface does *not* allow a thread to read the value of 
+// the semaphore directly -- even if you did read the value, the
+// only thing you would know is what the value used to be.  You don't
+// know what the value is now, because by the time you get the value
+// into a register, a context switch might have occurred,
+// and some other thread might have called P or V, so the true value might
+// now be different.
+
+class Semaphore:public dontcopythis
+{
+  public:
+    Semaphore (const char *debugName, int initialValue);	// set initial value
+     ~Semaphore ();		// de-allocate semaphore
+    const char *getName ()
+    {
+	return name;
+    }				// debugging assist
+
+    void P ();			// these are the only operations on a semaphore
+    void V ();			// they are both *atomic*
+
+  private:
+    const char *name;		// useful for debugging
+    int value;			// semaphore value, always >= 0
+    List *queue;		// threads waiting in P() for the value to be > 0
+};
+
+// 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:public dontcopythis
+{
+  public:
+    Lock (const char *debugName);	// initialize lock to be FREE
+     ~Lock ();			// deallocate lock
+    const 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.
+
+  private:
+    const char *name;		// for debugging
+    // plus some other stuff you'll need to define
+};
+
+// 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:public dontcopythis
+{
+  public:
+    Condition (const char *debugName);	// initialize condition to 
+    // "no one waiting"
+     ~Condition ();		// deallocate the condition
+    const 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
+
+  private:
+    const char *name;
+    // plus some other stuff you'll need to define
+};
+#endif // SYNCH_H