LockSupport中的park與unpark原理

LockSupport是用來創建locks的基本線程阻塞基元，比如AQS中實現線程掛起的方法，就是park,對應喚醒就是unpark。JDK中有使用的如下

LockSupport提供的是一個許可，如果存在許可，線程在調用park的時候，會立馬返回，此時許可也會被消費掉，如果沒有許可，則會阻塞。調用unpark的時候，如果許可本身不可用，則會使得許可可用

許可只有一個，不可累加

park源碼跟蹤

park的聲明形式有一下兩大塊

一部分多了一個Object參數，作爲blocker,另外的則沒有。blocker的好處在於，在診斷問題的時候能夠知道park的原因

推薦使用帶有Object的park操作

park函數作用

park用於掛起當前線程，如果許可可用，會立馬返回，並消費掉許可。

• park(Object): 恢復的條件爲 1：線程調用了unpark; 2:其它線程中斷了線程；3：發生了不可預料的事情
• parkNanos(Object blocker, long nanos):恢復的條件爲 1：線程調用了unpark; 2:其它線程中斷了線程；3：發生了不可預料的事情;4:過期時間到了
• parkUntil(Object blocker, long deadline):恢復的條件爲 1：線程調用了unpark; 2:其它線程中斷了線程；3：發生了不可預料的事情;4:指定的deadLine已經到了

以park的源碼爲例

public static void park(Object blocker) {
   //獲取當前線程
    Thread t = Thread.currentThread();
   //記錄當前線程阻塞的原因,底層就是unsafe.putObject,就是把對象存儲起來
    setBlocker(t, blocker);
    //執行park
    unsafe.park(false, 0L);
   //線程恢復後，去掉阻塞原因
    setBlocker(t, null);
}

從源碼可以看到真實的實現均在 unsafe

unsafe.park

核心實現如下

JavaThread* thread=JavaThread::thread_from_jni_environment(env);
...
thread->parker()->park(isAbsolute != 0, time);

就是獲取java線程的parker對象,然後執行它的park方法。Parker的定義如下

class Parker : public os::PlatformParker {
private:
   //表示許可
  volatile int _counter ; 
  Parker * FreeNext ;
  JavaThread * AssociatedWith ; // Current association
public:
  Parker() : PlatformParker() {
    //初始化_counter
    _counter       = 0 ; 
    FreeNext       = NULL ;
    AssociatedWith = NULL ;
  }
protected:
  ~Parker() { ShouldNotReachHere(); }
public:
  void park(bool isAbsolute, jlong time);
  void unpark();

  // Lifecycle operators  
  static Parker * Allocate (JavaThread * t) ;
  static void Release (Parker * e) ;
private:
  static Parker * volatile FreeList ;
  static volatile int ListLock ;

};

它繼承了os::PlatformParker，內置了一個volatitle的 _counter。PlatformParker則是在不同的操作系統中有不同的實現，以linux爲例

class PlatformParker : public CHeapObj {
  protected:
    //互斥變量類型
    pthread_mutex_t _mutex [1] ; 
   //條件變量類型
    pthread_cond_t  _cond  [1] ;

  public:        
     ~PlatformParker() { guarantee (0, "invariant") ; }

  public:
    PlatformParker() {
      int status;
     //初始化條件變量，使用    pthread_cond_t之前必須先執行初始化
      status = pthread_cond_init (_cond, NULL);
      assert_status(status == 0, status, "cond_init”);
      // 初始化互斥變量，使用    pthread_mutex_t之前必須先執行初始化
      status = pthread_mutex_init (_mutex, NULL);
      assert_status(status == 0, status, "mutex_init");
    }
} 

上述代碼均爲POSIX線程接口使用，所以pthread指的也就是posixThread

parker實現如下

void Parker::park(bool isAbsolute, jlong time) {
  if (_counter > 0) {
       //已經有許可了，用掉當前許可
      _counter = 0 ;
     //使用內存屏障，確保 _counter賦值爲0(寫入操作)能夠被內存屏障之後的讀操作獲取內存屏障事前的結果，也就是能夠正確的讀到0
      OrderAccess::fence();
     //立即返回
      return ;
  }

  Thread* thread = Thread::current();
  assert(thread->is_Java_thread(), "Must be JavaThread");
  JavaThread *jt = (JavaThread *)thread;

 if (Thread::is_interrupted(thread, false)) {
 // 線程執行了中斷，返回
    return;
  }

  if (time < 0 || (isAbsolute && time == 0) ) { 
    //時間到了，或者是代表絕對時間，同時絕對時間是0（此時也是時間到了），直接返回，java中的parkUtil傳的就是絕對時間，其它都不是
   return;
  }
  if (time > 0) {
  //傳入了時間參數，將其存入absTime，並解析成absTime->tv_sec(秒)和absTime->tv_nsec(納秒)存儲起來，存的是絕對時間
    unpackTime(&absTime, isAbsolute, time);
  }

 //進入safepoint region，更改線程爲阻塞狀態
  ThreadBlockInVM tbivm(jt);

 if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
  //如果線程被中斷，或者是在嘗試給互斥變量加鎖的過程中，加鎖失敗，比如被其它線程鎖住了，直接返回
    return;
  }
//這裏表示線程互斥變量鎖成功了
  int status ;
  if (_counter > 0)  {
    // 有許可了，返回
    _counter = 0;
    //對互斥變量解鎖
    status = pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
    OrderAccess::fence();
    return;
  }

#ifdef ASSERT
  // Don't catch signals while blocked; let the running threads have the signals.  
// (This allows a debugger to break into the running thread.)  
 //debug用
sigset_t oldsigs;
  sigset_t* allowdebug_blocked = os::Linux::allowdebug_blocked_signals();
  pthread_sigmask(SIG_BLOCK, allowdebug_blocked, &oldsigs);
#endif
//將java線程所擁有的操作系統線程設置成 CONDVAR_WAIT狀態 ，表示在等待某個條件的發生
OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);
//將java的_suspend_equivalent參數設置爲true
  jt->set_suspend_equivalent();
  // cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()
  if (time == 0) {
    //把調用線程放到等待條件的線程列表上，然後對互斥變量解鎖，（這兩是原子操作），這個時候線程進入等待，當它返回時，互斥變量再次被鎖住。
  //成功返回0，否則返回錯誤編號
    status = pthread_cond_wait (_cond, _mutex) ;
  } else {
  //同pthread_cond_wait，只是多了一個超時，如果超時還沒有條件出現，那麼重新獲取胡吃兩然後返回錯誤碼 ETIMEDOUT
    status = os::Linux::safe_cond_timedwait (_cond, _mutex, &absTime) ;
    if (status != 0 && WorkAroundNPTLTimedWaitHang) {
   //WorkAroundNPTLTimedWaitHang 是JVM的運行參數，默認爲1
  //去除初始化
      pthread_cond_destroy (_cond) ;
//重新初始化
      pthread_cond_init    (_cond, NULL);
    }
  }
  assert_status(status == 0 || status == EINTR ||
                status == ETIME || status == ETIMEDOUT,
                status, "cond_timedwait");

#ifdef ASSERT
  pthread_sigmask(SIG_SETMASK, &oldsigs, NULL);
#endif
 //等待結束後，許可被消耗，改爲0  _counter = 0 ;
//釋放互斥量的鎖
  status = pthread_mutex_unlock(_mutex) ;
  assert_status(status == 0, status, "invariant") ;
  // If externally suspended while waiting, re-suspend 
    if (jt->handle_special_suspend_equivalent_condition()) {
    jt->java_suspend_self();
  }
//加入內存屏障指令
  OrderAccess::fence();
}

從park的實現可以看到

1. 無論是什麼情況返回，park方法本身都不會告知調用方返回的原因，所以調用的時候一般都會去判斷返回的場景，根據場景做不同的處理
2. 線程的等待與掛起、喚醒等等就是使用的POSIX的線程API
3. park的許可通過原子變量_count實現，當被消耗時，_count爲0，只要擁有許可，就會立即返回

OrderAccess::fence();

在linux中實現原理如下

inline void OrderAccess::fence() {
  if (os::is_MP()) {
#ifdef AMD64
  // 沒有使用mfence,因爲mfence有時候性能差於使用 locked addl
    __asm__ volatile ("lock; addl $0,0(%%rsp)" : : : "cc", "memory");
#else    __asm__ volatile ("lock; addl $0,0(%%esp)" : : : "cc", "memory");
#endif  }
}

內存重排序網上的驗證

ThreadBlockInVM tbivm(jt)

這屬於C++新建變量的語法，也就是調用構造函數新建了一個變量，變量名爲tbivm,參數爲jt。類的實現爲

class ThreadBlockInVM : public ThreadStateTransition {
 public:
  ThreadBlockInVM(JavaThread *thread)
  : ThreadStateTransition(thread) {
    // Once we are blocked vm expects stack to be walkable    
    thread->frame_anchor()->make_walkable(thread);
   //把線程由運行狀態轉成阻塞狀態
    trans_and_fence(_thread_in_vm, _thread_blocked);
  }
  ...
};

_thread_in_vm 表示線程當前在VM中執行，_thread_blocked表示線程當前阻塞了，他們是globalDefinitions.hpp中定義的枚舉

//這個枚舉是用來追蹤線程在代碼的那一塊執行，用來給 safepoint code使用，有4種重要的類型，_thread_new/_thread_in_native/_thread_in_vm/_thread_in_Java。形如xxx_trans的狀態都是中間狀態，表示線程正在由一種狀態變成另一種狀態，這種方式使得 safepoint code在處理線程狀態時，不需要對線程進行掛起，使得safe point code運行更快，而給定一個狀態，通過+1就可以得到他的轉換狀態
enum JavaThreadState {
  _thread_uninitialized     =  0, // should never happen (missing initialization) 
_thread_new               =  2, // just starting up, i.e., in process of being initialized 
_thread_new_trans         =  3, // corresponding transition state (not used, included for completeness)  
_thread_in_native         =  4, // running in native code  . This is a safepoint region, since all oops will be in jobject handles
_thread_in_native_trans   =  5, // corresponding transition state  
_thread_in_vm             =  6, // running in VM 
_thread_in_vm_trans       =  7, // corresponding transition state 
_thread_in_Java           =  8, //  Executing either interpreted or compiled Java code running in Java or in stub code  
_thread_in_Java_trans     =  9, // corresponding transition state (not used, included for completeness) 
_thread_blocked           = 10, // blocked in vm 
_thread_blocked_trans     = 11, // corresponding transition state 
_thread_max_state         = 12  // maximum thread state+1 - used for statistics allocation
};

父類ThreadStateTransition中定義trans_and_fence如下

void trans_and_fence(JavaThreadState from, JavaThreadState to) { transition_and_fence(_thread, from, to);} //_thread即構造函數傳進來de thread
// transition_and_fence must be used on any thread state transition
// where there might not be a Java call stub on the stack, in
// particular on Windows where the Structured Exception Handler is
// set up in the call stub. os::write_memory_serialize_page() can
// fault and we can't recover from it on Windows without a SEH in
// place.
//transition_and_fence方法必須在任何線程狀態轉換的時候使用
static inline void transition_and_fence(JavaThread *thread, JavaThreadState from, JavaThreadState to) {
  assert(thread->thread_state() == from, "coming from wrong thread state");
  assert((from & 1) == 0 && (to & 1) == 0, "odd numbers are transitions states");
//標識線程轉換中
    thread->set_thread_state((JavaThreadState)(from + 1));

  // 設置內存屏障，確保新的狀態能夠被VM 線程看到
if (os::is_MP()) {
    if (UseMembar) {
      // Force a fence between the write above and read below     
        OrderAccess::fence();
    } else {
      // Must use this rather than serialization page in particular on Windows      
        InterfaceSupport::serialize_memory(thread);
    }
  }

  if (SafepointSynchronize::do_call_back()) {
    SafepointSynchronize::block(thread);
  }
//線程狀態轉換成最終的狀態,對待這裏的場景就是阻塞
  thread->set_thread_state(to);

  CHECK_UNHANDLED_OOPS_ONLY(thread->clear_unhandled_oops();)
}

操作系統線程狀態的一般取值

在osThread中給定了操作系統線程狀態的大致取值，它本身是依據平臺而定

enum ThreadState {
 ALLOCATED,                    // Memory has been allocated but not initialized  
INITIALIZED,                  // The thread has been initialized but yet started 
RUNNABLE,                     // Has been started and is runnable, but not necessarily running  
MONITOR_WAIT,                 // Waiting on a contended monitor lock  
CONDVAR_WAIT,                 // Waiting on a condition variable  
OBJECT_WAIT,                  // Waiting on an Object.wait() call  
BREAKPOINTED,                 // Suspended at breakpoint  
SLEEPING,                     // Thread.sleep()  
ZOMBIE                        // All done, but not reclaimed yet
};

unpark 源碼追蹤

實現如下

void Parker::unpark() {
  int s, status ;
 //給互斥量加鎖，如果互斥量已經上鎖，則阻塞到互斥量被解鎖
//park進入wait時，_mutex會被釋放
  status = pthread_mutex_lock(_mutex);
  assert (status == 0, "invariant") ; 
  //存儲舊的_counter
  s = _counter; 
//許可改爲1，每次調用都設置成發放許可
  _counter = 1;
  if (s < 1) {
     //之前沒有許可
     if (WorkAroundNPTLTimedWaitHang) {
      //默認執行 ,釋放信號，表明條件已經滿足，將喚醒等待的線程
        status = pthread_cond_signal (_cond) ;
        assert (status == 0, "invariant") ;
        //釋放鎖
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant") ;
     } else {
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant") ;
        status = pthread_cond_signal (_cond) ;
        assert (status == 0, "invariant") ;
     }
  } else {
   //一直有許可，釋放掉自己加的鎖,有許可park本身就返回了
    pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
  }
}

從源碼可知unpark本身就是發放許可，並通知等待的線程，已經可以結束等待了

總結

• park/unpark能夠精準的對線程進行喚醒和等待。
• linux上的實現是通過POSIX的線程API的等待、喚醒、互斥、條件來進行實現的
• park在執行過程中首選看是否有許可，有許可就立馬返回，而每次unpark都會給許可設置成有，這意味着，可以先執行unpark，給予許可，再執行park立馬自行，適用於producer快，而consumer還未完成的場景參考地址