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

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：

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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。

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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。

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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。

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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。

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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。

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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。

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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。

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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 threadsif (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));}}
#endifassert(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 regionsGCLocker::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 ASSERTfor (; 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 finishassert(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 stackreturn !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 sectionSafepointSynchronize::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;
}