// synch.cc 
//  Routines for synchronizing threads.  Three kinds of
//  synchronization routines are defined here: semaphores, locks 
//      and condition variables (the implementation of the last two
//  are left to the reader).
//
// Any implementation of a synchronization routine needs some
// primitive atomic operation.  We assume Nachos is running on
// a uniprocessor, and thus atomicity can be provided by
// turning off interrupts.  While interrupts are disabled, no
// context switch can occur, and thus the current thread is guaranteed
// to hold the CPU throughout, until interrupts are reenabled.
//
// Because some of these routines might be called with interrupts
// already disabled (Semaphore::V for one), instead of turning
// on interrupts at the end of the atomic operation, we always simply
// re-set the interrupt state back to its original value (whether
// that be disabled or enabled).
//
// Copyright (c) 1992-1993 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// of liability and disclaimer of warranty provisions.

#include "copyright.h"
#include "synch-sleep.h"
#include "system.h"

//----------------------------------------------------------------------
// 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!
//----------------------------------------------------------------------

Semaphore::~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.
//----------------------------------------------------------------------

void
Semaphore::P()
{
    IntStatus oldLevel = interrupt->SetLevel(IntOff);   // disable interrupts
    
    while (value == 0) {            // semaphore not available
    queue->Append((void *)currentThread);   // so go to sleep
    currentThread->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.
//----------------------------------------------------------------------

void
Semaphore::V()
{
    Thread *thread;
    IntStatus oldLevel = interrupt->SetLevel(IntOff);

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

// Dummy functions -- so we can compile our later assignments 
// Note -- without a correct implementation of Condition::Wait(), 
// the test case in the network assignment won't work!

//构造函数，初始化变量
Lock::Lock(char* debugName)
{
    name = debugName;
    lock_flag = FREE;
    waitlock_queue = new List();
    haslock_thread = NULL;
}

//析构函数
Lock::~Lock()
{
    delete waitlock_queue;
}

//当前线程获得锁的函数
void Lock::Acquire()
{   
    IntStatus oldLevel = interrupt->SetLevel(IntOff); // disable interrupts
    while(lock_flag == BUSY)    //wait until the lock is FREE
    {
        waitlock_queue->Append( (void *)currentThread );//将当前线程插入到等待队列中
        currentThread->Sleep(); //当前线程进入睡眠
    }
    lock_flag = BUSY; //then set it to BUSY
    haslock_thread = currentThread;//当前线程拥有锁
    (void) interrupt->SetLevel(oldLevel);   // re-enable interrupts
}

//释放锁
void Lock::Release()        //把线程 从堵塞在锁队列 里面的换到  就绪队列里面
{
    Thread *ready_thread;
    IntStatus oldLevel = interrupt->SetLevel(IntOff); // disable interrupts
    ASSERT(isHeldByCurrentThread() );//检查当前线程是否拥有锁
    lock_flag = FREE; //set lock to be FREE
    haslock_thread = NULL;
    ready_thread = (Thread *)waitlock_queue->Remove(); //waking up a thread waiting in Acquire
    if(ready_thread != NULL)
        scheduler->ReadyToRun(ready_thread);
    (void) interrupt->SetLevel(oldLevel);   // re-enable interrupts
}

//检查当前线程是否拥有锁
bool Lock::isHeldByCurrentThread()
{
    if( lock_flag == BUSY && haslock_thread == currentThread )
        return true;
    else
        return false;
}

//构造函数，初始化变量
Condition::Condition(char* debugName)
{
    name = debugName;
    waitcondition_queue = new List();
}

Condition::~Condition()
{
    delete waitcondition_queue;
}

//线程等待条件变量
void Condition::Wait(Lock* conditionLock)       //传锁主要是为了判断当前锁是否有锁住东西与锁里面的队列无关
{
    Thread *thread;
    IntStatus oldLevel = interrupt->SetLevel(IntOff); // disable interrupts  保存以前的中断向量
            //因为中断是以随机方式发生， 从而有可能抢占刚刚获得lock的中断处理程序。
            //如果抢占的中断程序也刚好需要这把锁， 或者说是其后抢占的内核程序需要这把锁， 而这把锁没有释放， 从而死锁
        //如果系统中存在死锁，对系统的影响将是显而易见的。这预示着操作系统的宝贵资源被无法完成的任务占据。
        //所以检测和预防死锁是非常必要的。但是由于操作系统的不确定性，死锁的检测和预防将是非常复杂的工作
    thread = currentThread;
    ASSERT(conditionLock->isHeldByCurrentThread() );//所有的这些操作必须在当前线程获得一个锁的前提下    也就是当前线程里面的锁变量 已经获得锁。
    conditionLock->Release(); //release the lock         释放锁
    waitcondition_queue->Append((void *)thread);//当前线程被放进条件变量的队列
    thread->Sleep(); //进入睡眠状态,relinquish the CPU
    conditionLock->Acquire(); //re-acquire the lock    当当前线程再次获得锁时  一般是表示 已经有生产了。   但如何确定是消费者运行后还是生产者运行后再次获得呢  
        //因为只有signal才能再次把该线程从条件变量里面的锁解出来    当获得锁时也表示当前线程已经不在锁的等待队列里面，只可能被唤到条件队列或者 直接执行完
                                        
    (void) interrupt->SetLevel(oldLevel);   // re-enable interrupts  恢复中断
}

//唤醒一个等待该条件变量的线程
void Condition::Signal(Lock* conditionLock)
{
    Thread* thread_ready;
    IntStatus oldLevel = interrupt->SetLevel(IntOff); // disable interrupts
    thread_ready = (Thread *)waitcondition_queue->Remove();  //移走条件变量里面的线程
    ASSERT(conditionLock->isHeldByCurrentThread() );
    if(thread_ready != NULL)
        scheduler->ReadyToRun(thread_ready);                          //将一个线程从 阻塞队列里面 换到就绪队列里面
    (void) interrupt->SetLevel(oldLevel);   // re-enable interrupts
}

//唤醒所有等待该条件的线程
void Condition::Broadcast(Lock* conditionLock)
{
    Thread *thread;
    IntStatus oldLevel = interrupt->SetLevel(IntOff); // disable interrupts
    while( (thread = (Thread *)waitcondition_queue->Remove() ) != NULL)
        scheduler->ReadyToRun(thread);
    (void) interrupt->SetLevel(oldLevel);   // re-enable interrupts
}

线程安全的Table类

#include "Table.h"

Table::Table(int size)
// create a table to hold at most 'size' entries.
{
     S = size;
     lock = new Lock("Tablelock");
     L = new int[size];
     value = new int[size];
     for ( int i=0; i<size; i++ )
         value[i] = 0;           //初始化都为0
}

Table::~Table() //析构函数 
{
     for ( int i=0; i<=S; i++ ){
         Release(i);
     delete lock;
     delete value;
}

int 
Table::Alloc(void *object)
// allocate a table slot for 'object'.
// return the table index for the slot or -1 on error.
{
      int i;
      lock->Acquire();
      for ( i=0; i<S; i++ )
          if ( value[i] == 0 )
          {
               value[i] = 1;
               object = (void *)(L+i);//应该反过来才对吧！
               break;
          }
      lock->Release();
      if ( i<S )
           return i+1;//具体的位置应该是数组下标加1
      else 
           return -1;
}

void *Table::Get(int *index)
// return the object from table index 'index' or NULL on error.
// (assert index is in range).  Leave the table entry allocated
// and the pointer in place.
{
     if ( index>0 && index<=S )
          return (void *)(L+index-1);
     else
          return NULL;
}

void Table::Release(int index)  // free a table slot
{
     lokc->Acquire();
     if ( index>0 && index<=S )
          delete (L+index-1);   //同时应该把该值 对应的value 数组进行修改吧！
     lock->Release();
}

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