jvm安全点(五)openjdk17 c++源码垃圾回收之安全点阻塞状态线程在安全点同步中无需调用block函数的详细流程解析

发布于：2025-05-23 ⋅ 阅读:(21) ⋅ 点赞:(0)

关于阻塞状态线程在安全点同步中无需调用block函数的详细流程解析：

1. 安全点同步入口：SafepointSynchronize::begin()

VM线程调用此函数启动安全点，核心步骤如下：

  • 获取线程锁（Threads_lock）：防止新线程启动或旧线程退出。

  • 初始化计数器：_waiting_to_block设为当前线程总数，表示需等待阻塞的线程数。

  • 激活安全点（arm_safepoint()）：设置全局安全点标志，触发线程轮询。

  • 调用synchronize_threads()：进入主循环，等待所有线程进入安全点。

2. 同步线程主循环：synchronize_threads()

此函数通过多次迭代检查所有线程状态，直到所有线程进入安全点：

  • 首次遍历构建链表：将所有未进入安全点的线程（thread_not_running(cur_tss) == false）加入链表tss_head。

  • 主循环处理未阻塞线程：

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    do {
  // 遍历链表，检查每个线程状态
  while (cur_tss != NULL) {
    if (thread_not_running(cur_tss)) {
      // 从链表中移除该线程，减少still_running计数
      --still_running;
      ...
    } else {
      // 保留在链表中，继续处理
      ...
    }
  }
} while (still_running > 0);

    • 关键点：只有状态仍为运行的线程会保留在链表中，后续可能触发阻塞操作。

3. 状态检查逻辑：thread_not_running()

该函数判断线程是否已处于安全点或无需阻塞：

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bool SafepointSynchronize::thread_not_running(ThreadSafepointState *cur_state) {
  if (!cur_state->is_running()) return true; // 已非运行
  cur_state->examine_state_of_thread(...);   // 检查线程状态
  return !cur_state->is_running();           // 检查后再次确认
}

  • examine_state_of_thread()：深入分析线程状态，决定是否需要阻塞。

4. 线程状态分析：examine_state_of_thread()

此函数通过safepoint_safe_with()判断线程是否已安全：

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void ThreadSafepointState::examine_state_of_thread(...) {
  JavaThreadState stable_state;
  if (!try_stable_load_state(&stable_state, ...)) return; // 无法获取稳定状态，视为运行中
  
  if (safepoint_safe_with(_thread, stable_state)) {
    account_safe_thread(); // 标记线程为安全
    return;
  }
  // 其他状态需继续运行，直到主动挂起
}

  • safepoint_safe_with()：核心判断逻辑，针对阻塞状态线程。

5. 阻塞状态判定：safepoint_safe_with()

此函数明确处理阻塞状态：

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static bool safepoint_safe_with(JavaThread *thread, JavaThreadState state) {
  switch(state) {
    case _thread_blocked:
      // 阻塞状态的线程直接视为安全
      return true;
    case _thread_in_native:
      // 本地方法线程需检查栈可遍历性
      return ...;
    default:
      return false; // 其他状态需进一步处理
  }
}

  • 阻塞状态（_thread_blocked）：直接返回true，无需额外操作。

6. 标记安全线程：account_safe_thread()

当线程被判定为安全时，此函数更新全局状态：

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void ThreadSafepointState::account_safe_thread() {
  SafepointSynchronize::decrement_waiting_to_block(); // 减少等待计数器
  if (_thread->in_critical()) {
    SafepointSynchronize::increment_jni_active_count(); // 处理JNI临界区
  }
  _safepoint_safe = true; // 标记线程为安全点安全
}

  • 减少_waiting_to_block：表示该线程已处理完毕。

  • 标记_safepoint_safe：后续检查中is_running()将返回false。

7. 阻塞函数的调用条件

在提供的代码中，未直接调用block()函数，但隐含逻辑如下：

  • 需主动阻塞的线程：状态为运行中（如执行Java代码、JNI非临界区），会保留在tss_head链表中，等待后续处理（如轮询触发挂起）。

  • 阻塞状态的线程：因safepoint_safe_with()返回true，被移出链表，不会进入需阻塞的分支。

总结：为何阻塞状态线程无需调用block

  1. 状态判定：通过safepoint_safe_with()识别阻塞状态，直接标记为安全。

  2. 链表移除：在synchronize_threads()循环中，安全线程被移出待处理链表。

  3. 计数器更新：_waiting_to_block递减，最终为0时同步完成。

  4. 避免冗余操作：已阻塞线程无需二次挂起，提升效率。

此机制确保安全点同步仅处理必要线程，避免对已阻塞线程的不必要操作。

##源码

// Roll all threads forward to a safepoint and suspend them all
void SafepointSynchronize::begin() {
  assert(Thread::current()->is_VM_thread(), "Only VM thread may execute a safepoint");

  EventSafepointBegin begin_event;
  SafepointTracing::begin(VMThread::vm_op_type());

  Universe::heap()->safepoint_synchronize_begin();

  // By getting the Threads_lock, we assure that no threads are about to start or
  // exit. It is released again in SafepointSynchronize::end().
  Threads_lock->lock();

  assert( _state == _not_synchronized, "trying to safepoint synchronize with wrong state");

  int nof_threads = Threads::number_of_threads();

  _nof_threads_hit_polling_page = 0;

  log_debug(safepoint)("Safepoint synchronization initiated using %s wait barrier. (%d threads)", _wait_barrier->description(), nof_threads);

  // Reset the count of active JNI critical threads
  _current_jni_active_count = 0;

  // Set number of threads to wait for
  _waiting_to_block = nof_threads;

  jlong safepoint_limit_time = 0;
  if (SafepointTimeout) {
    // Set the limit time, so that it can be compared to see if this has taken
    // too long to complete.
    safepoint_limit_time = SafepointTracing::start_of_safepoint() + (jlong)SafepointTimeoutDelay * (NANOUNITS / MILLIUNITS);
    timeout_error_printed = false;
  }

  EventSafepointStateSynchronization sync_event;
  int initial_running = 0;

  // Arms the safepoint, _current_jni_active_count and _waiting_to_block must be set before.
  arm_safepoint();

  // Will spin until all threads are safe.
  int iterations = synchronize_threads(safepoint_limit_time, nof_threads, &initial_running);
  assert(_waiting_to_block == 0, "No thread should be running");

#ifndef PRODUCT
  // Mark all threads
  if (VerifyCrossModifyFence) {
    JavaThreadIteratorWithHandle jtiwh;
    for (; JavaThread *cur = jtiwh.next(); ) {
      cur->set_requires_cross_modify_fence(true);
    }
  }

  if (safepoint_limit_time != 0) {
    jlong current_time = os::javaTimeNanos();
    if (safepoint_limit_time < current_time) {
      log_warning(safepoint)("# SafepointSynchronize: Finished after "
                    INT64_FORMAT_W(6) " ms",
                    (int64_t)(current_time - SafepointTracing::start_of_safepoint()) / (NANOUNITS / MILLIUNITS));
    }
  }
#endif

  assert(Threads_lock->owned_by_self(), "must hold Threads_lock");

  // Record state
  _state = _synchronized;

  OrderAccess::fence();

  // Set the new id
  ++_safepoint_id;

#ifdef ASSERT
  // Make sure all the threads were visited.
  for (JavaThreadIteratorWithHandle jtiwh; JavaThread *cur = jtiwh.next(); ) {
    assert(cur->was_visited_for_critical_count(_safepoint_counter), "missed a thread");
  }
#endif // ASSERT

  // Update the count of active JNI critical regions
  GCLocker::set_jni_lock_count(_current_jni_active_count);

  post_safepoint_synchronize_event(sync_event,
                                   _safepoint_id,
                                   initial_running,
                                   _waiting_to_block, iterations);

  SafepointTracing::synchronized(nof_threads, initial_running, _nof_threads_hit_polling_page);

  // We do the safepoint cleanup first since a GC related safepoint
  // needs cleanup to be completed before running the GC op.
  EventSafepointCleanup cleanup_event;
  do_cleanup_tasks();
  post_safepoint_cleanup_event(cleanup_event, _safepoint_id);

  post_safepoint_begin_event(begin_event, _safepoint_id, nof_threads, _current_jni_active_count);
  SafepointTracing::cleanup();
}



int SafepointSynchronize::synchronize_threads(jlong safepoint_limit_time, int nof_threads, int* initial_running)
{
  JavaThreadIteratorWithHandle jtiwh;

#ifdef ASSERT
  for (; JavaThread *cur = jtiwh.next(); ) {
    assert(cur->safepoint_state()->is_running(), "Illegal initial state");
  }
  jtiwh.rewind();
#endif // ASSERT

  // Iterate through all threads until it has been determined how to stop them all at a safepoint.
  int still_running = nof_threads;
  ThreadSafepointState *tss_head = NULL;
  ThreadSafepointState **p_prev = &tss_head;
  for (; JavaThread *cur = jtiwh.next(); ) {
    ThreadSafepointState *cur_tss = cur->safepoint_state();
    assert(cur_tss->get_next() == NULL, "Must be NULL");
    if (thread_not_running(cur_tss)) {
      --still_running;
    } else {
      *p_prev = cur_tss;
      p_prev = cur_tss->next_ptr();
    }
  }
  *p_prev = NULL;

  DEBUG_ONLY(assert_list_is_valid(tss_head, still_running);)

  *initial_running = still_running;

  // If there is no thread still running, we are already done.
  if (still_running <= 0) {
    assert(tss_head == NULL, "Must be empty");
    return 1;
  }

  int iterations = 1; // The first iteration is above.
  int64_t start_time = os::javaTimeNanos();

  do {
    // Check if this has taken too long:
    if (SafepointTimeout && safepoint_limit_time < os::javaTimeNanos()) {
      print_safepoint_timeout();
    }

    p_prev = &tss_head;
    ThreadSafepointState *cur_tss = tss_head;
    while (cur_tss != NULL) {
      assert(cur_tss->is_running(), "Illegal initial state");
      if (thread_not_running(cur_tss)) {
        --still_running;
        *p_prev = NULL;
        ThreadSafepointState *tmp = cur_tss;
        cur_tss = cur_tss->get_next();
        tmp->set_next(NULL);
      } else {
        *p_prev = cur_tss;
        p_prev = cur_tss->next_ptr();
        cur_tss = cur_tss->get_next();
      }
    }

    DEBUG_ONLY(assert_list_is_valid(tss_head, still_running);)

    if (still_running > 0) {
      back_off(start_time);
    }

    iterations++;
  } while (still_running > 0);

  assert(tss_head == NULL, "Must be empty");

  return iterations;
}

bool SafepointSynchronize::thread_not_running(ThreadSafepointState *cur_state) {
  if (!cur_state->is_running()) {
    return true;
  }
  cur_state->examine_state_of_thread(SafepointSynchronize::safepoint_counter());
  if (!cur_state->is_running()) {
    return true;
  }
  LogTarget(Trace, safepoint) lt;
  if (lt.is_enabled()) {
    ResourceMark rm;
    LogStream ls(lt);
    cur_state->print_on(&ls);
  }
  return false;
}

void ThreadSafepointState::examine_state_of_thread(uint64_t safepoint_count) {
  assert(is_running(), "better be running or just have hit safepoint poll");

  JavaThreadState stable_state;
  if (!SafepointSynchronize::try_stable_load_state(&stable_state, _thread, safepoint_count)) {
    // We could not get stable state of the JavaThread.
    // Consider it running and just return.
    return;
  }

  if (safepoint_safe_with(_thread, stable_state)) {
    std::cout << "@@@@yym%%%%----myThreadName----safepoint_safe_with----" << _thread->name() << std::endl;
    account_safe_thread();
    return;
  }

  // All other thread states will continue to run until they
  // transition and self-block in state _blocked
  // Safepoint polling in compiled code causes the Java threads to do the same.
  // Note: new threads may require a malloc so they must be allowed to finish

  assert(is_running(), "examine_state_of_thread on non-running thread");
  return;
}

static bool safepoint_safe_with(JavaThread *thread, JavaThreadState state) {
  switch(state) {
  case _thread_in_native:
    // native threads are safe if they have no java stack or have walkable stack
    return !thread->has_last_Java_frame() || thread->frame_anchor()->walkable();

  case _thread_blocked:
    // On wait_barrier or blocked.
    // Blocked threads should already have walkable stack.
    assert(!thread->has_last_Java_frame() || thread->frame_anchor()->walkable(), "blocked and not walkable");
    return true;

  default:
    return false;
  }
}

void ThreadSafepointState::account_safe_thread() {
  SafepointSynchronize::decrement_waiting_to_block();
  if (_thread->in_critical()) {
    // Notice that this thread is in a critical section
    SafepointSynchronize::increment_jni_active_count();
  }
  DEBUG_ONLY(_thread->set_visited_for_critical_count(SafepointSynchronize::safepoint_counter());)
  assert(!_safepoint_safe, "Must be unsafe before safe");
  _safepoint_safe = true;
}

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