溫故知新-多線程-深入刨析park、unpark

---

– Posted by 微博@Yangsc_o

• 原創文章，版權聲明：自由轉載-非商用-非衍生-保持署名 | Creative Commons BY-NC-ND 3.0

---

摘要

本文主要介紹park、unpark的功能以及hotspot實現原理，爲下一篇介紹ReentrantLock做鋪墊！

park、unpark

LockSupport類是Java6引入的一個類，提供了基本的線程同步原語。LockSupport實際上是調用了Unsafe類裏的函數，歸結到Unsafe裏，兩個函數但爲上層提供了強大的同步原語。

public native void unpark(Thread jthread);
public native void park(boolean isAbsolute, long time);
// isAbsolute參數是指明時間是絕對的，還是相對的。

unpark函數爲線程提供“許可(permit)”，線程調用park函數則等待“許可”。這個有點像信號量，但是這個“許可”是不能疊加的，“許可”是一次性的。

比如線程B連續調用了三次unpark函數，當線程A調用park函數就使用掉這個“許可”，如果線程A再次調用park，則進入等待狀態。

看一下hotspot實現

在Parker類裏的_counter字段，就是用來記錄所謂的“許可”的。

位置：hotspot/src/share/vm/runtime/park.hpp

簡化代碼如下

class Parker : public os::PlatformParker {
private:
  volatile int _counter ;
  Parker * FreeNext ;
  JavaThread * AssociatedWith ; // Current association
public:
  Parker() : PlatformParker() {
    _counter       = 0 ;
    FreeNext       = NULL ;
    AssociatedWith = NULL ;
  }
public:
  // For simplicity of interface with Java, all forms of park (indefinite,
  // relative, and absolute) are multiplexed into one call.
  void park(bool isAbsolute, jlong time);
  void unpark();
};

• park

  1. 簡化代碼如下：當調用park時，先嚐試直接能否直接拿到“許可”，即_counter>0時，如果成功，則把_counter設置爲0,並返回;

  2. 如果不成功，則構造一個ThreadBlockInVM，然後檢查_counter是不是>0，如果是，則把_counter設置爲0，unlock mutex並返回;

  3. Java dos裏提到，當下面三種情況下park函數會返回，在這段代碼裏也可以體現；

     Some other thread invokes unpark with the current thread as the target; or
     Some other thread interrupts the current thread; or
     The call spuriously (that is, for no reason) returns.

void Parker::park(bool isAbsolute, jlong time) {

  if (Atomic::xchg(0, &_counter) > 0) return;

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

  if (Thread::is_interrupted(thread, false)) {
    return;
  }
  // Next, demultiplex/decode time arguments
  struct timespec absTime;
  if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
    return;
  }
  if (time > 0) {
    unpackTime(&absTime, isAbsolute, time);
  }
  ... 
  if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
    return;
  }

  int status ;
  if (_counter > 0)  { // no wait needed
    _counter = 0;
    status = pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
    // Paranoia to ensure our locked and lock-free paths interact
    // correctly with each other and Java-level accesses.
    OrderAccess::fence();
    return;
  }

• unpark

unpark時，設置_counter爲1；如果_counter = 1，unlock mutext返回。如果_counter=0，需要調用pthread_cond_signal喚醒在park中等待的線程：

void Parker::unpark() {
  int s, status ;
  status = pthread_mutex_lock(_mutex);
  assert (status == 0, "invariant") ;
  s = _counter;
  _counter = 1;
  if (s < 1) {
     if (WorkAroundNPTLTimedWaitHang) {
        status = pthread_cond_signal (_cond) ;
        assert (status == 0, "invariant") ;
        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 {
    pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
  }
}

---

自此park、unpark已經分析完畢；

參考

Java的LockSupport.park()實現分析

---

你的鼓勵也是我創作的動力

打賞地址