1. VMThread::inner_execute() - 触发安全点​​

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void VMThread::inner_execute(VM_Operation* op) { if (op->evaluate_at_safepoint()) { SafepointSynchronize::begin(); // 进入安全点，阻塞所有线程 // ...执行GC等操作... SafepointSynchronize::end(); // 结束安全点，唤醒线程 } }

• ​​功能​​：执行需要安全点的 VM 操作（如 GC）。
• ​​关键点​​：
  • SafepointSynchronize::begin()：暂停所有线程，进入安全点。
  • SafepointSynchronize::end()：完成 GC 后，调用此函数解除线程阻塞。

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​​2. SafepointSynchronize::end() - 结束安全点​​

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void SafepointSynchronize::end() { disarm_safepoint(); // 核心：解除安全点 Universe::heap()->safepoint_synchronize_end(); // GC后清理 }

• ​​功能​​：安全点结束时的清理工作。
• ​​核心调用​​：disarm_safepoint() 负责恢复线程运行。

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​​3. SafepointSynchronize::disarm_safepoint() - 解除安全点​​

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void SafepointSynchronize::disarm_safepoint() { _state = _not_synchronized; // 全局状态标记为非同步 Atomic::store(&_safepoint_counter, _safepoint_counter + 1); // 递增安全点ID // 恢复所有线程状态 for (JavaThread *current : JavaThreadIterator()) { current->safepoint_state()->restart(); // 标记线程为运行状态 } _wait_barrier->disarm(); // 唤醒阻塞的线程 Threads_lock->unlock(); // 解锁线程列表 }

• ​​功能​​：
  • 将全局安全点状态设置为 ​​非同步​​。
  • 更新安全点计数器，触发内存屏障保证可见性。
  • 遍历所有线程，调用 restart() 重置线程状态。
  • 调用屏障的 disarm() 方法唤醒所有线程。

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​​4. LinuxWaitBarrier::disarm() - 唤醒线程​​

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void LinuxWaitBarrier::disarm() { _futex_barrier = 0; // 重置屏障值 syscall(SYS_futex, &_futex_barrier, FUTEX_WAKE_PRIVATE, INT_MAX); // 唤醒所有等待线程 }

• ​​功能​​：通过 Linux 的 futex 系统调用唤醒所有阻塞在安全点的线程。
• ​​关键点​​：
  • FUTEX_WAKE_PRIVATE：唤醒所有在 _futex_barrier 上等待的线程。
  • INT_MAX：唤醒最大数量的线程（实际唤醒所有等待的线程）。

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​​5. 线程阻塞与唤醒机制​​

• ​​线程阻塞​​：
  • 在安全点开始时，线程通过 SafepointSynchronize::block() 调用 futex 的 FUTEX_WAIT 进入阻塞状态。

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  void SafepointSynchronize::block(JavaThread* thread) { _wait_barrier->wait(active_safepoint_id); // FUTEX_WAIT }

• ​​线程唤醒​​：
  • GC 完成后，disarm_safepoint() 调用 LinuxWaitBarrier::disarm()，通过 FUTEX_WAKE 唤醒所有阻塞线程。

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​​总结​​

1. ​​安全点进入​​：GC 开始时，所有线程通过 futex 进入阻塞状态。
2. ​​GC 执行​​：VM 线程在安全点内执行垃圾回收。
3. ​​安全点退出​​：
   • 更新全局状态和计数器。
   • 重置每个线程的运行状态。
   • 调用 futex 的 FUTEX_WAKE 唤醒所有线程。
4. ​​线程恢复​​：被唤醒的线程继续执行后续代码。

这些代码是 ​​垃圾回收完成后解除线程阻塞的核心实现​​，通过操作系统提供的 futex 机制高效地管理线程的阻塞与唤醒。

 ##源码

void VMThread::inner_execute(VM_Operation* op) {assert(Thread::current()->is_VM_thread(), "Must be the VM thread");VM_Operation* prev_vm_operation = NULL;if (_cur_vm_operation != NULL) {// Check that the VM operation allows nested VM operation.// This is normally not the case, e.g., the compiler// does not allow nested scavenges or compiles.if (!_cur_vm_operation->allow_nested_vm_operations()) {fatal("Unexpected nested VM operation %s requested by operation %s",op->name(), _cur_vm_operation->name());}op->set_calling_thread(_cur_vm_operation->calling_thread());prev_vm_operation = _cur_vm_operation;}_cur_vm_operation = op;HandleMark hm(VMThread::vm_thread());EventMarkVMOperation em("Executing %sVM operation: %s", prev_vm_operation != NULL ? "nested " : "", op->name());log_debug(vmthread)("Evaluating %s %s VM operation: %s",prev_vm_operation != NULL ? "nested" : "",_cur_vm_operation->evaluate_at_safepoint() ? "safepoint" : "non-safepoint",_cur_vm_operation->name());bool end_safepoint = false;if (_cur_vm_operation->evaluate_at_safepoint() &&!SafepointSynchronize::is_at_safepoint()) {SafepointSynchronize::begin();if (_timeout_task != NULL) {_timeout_task->arm();}end_safepoint = true;}evaluate_operation(_cur_vm_operation);if (end_safepoint) {if (_timeout_task != NULL) {_timeout_task->disarm();}SafepointSynchronize::end();}_cur_vm_operation = prev_vm_operation;
}// Wake up all threads, so they are ready to resume execution after the safepoint
// operation has been carried out
void SafepointSynchronize::end() {assert(Threads_lock->owned_by_self(), "must hold Threads_lock");EventSafepointEnd event;assert(Thread::current()->is_VM_thread(), "Only VM thread can execute a safepoint");disarm_safepoint();Universe::heap()->safepoint_synchronize_end();SafepointTracing::end();post_safepoint_end_event(event, safepoint_id());
}void SafepointSynchronize::disarm_safepoint() {uint64_t active_safepoint_counter = _safepoint_counter;{JavaThreadIteratorWithHandle jtiwh;
#ifdef ASSERT// A pending_exception cannot be installed during a safepoint.  The threads// may install an async exception after they come back from a safepoint into// pending_exception after they unblock.  But that should happen later.for (; JavaThread *cur = jtiwh.next(); ) {assert (!(cur->has_pending_exception() &&cur->safepoint_state()->is_at_poll_safepoint()),"safepoint installed a pending exception");}
#endif // ASSERTOrderAccess::fence(); // keep read and write of _state from floating upassert(_state == _synchronized, "must be synchronized before ending safepoint synchronization");// Change state first to _not_synchronized.// No threads should see _synchronized when running._state = _not_synchronized;// Set the next dormant (even) safepoint id.assert((_safepoint_counter & 0x1) == 1, "must be odd");Atomic::release_store(&_safepoint_counter, _safepoint_counter + 1);OrderAccess::fence(); // Keep the local state from floating up.jtiwh.rewind();for (; JavaThread *current = jtiwh.next(); ) {// Clear the visited flag to ensure that the critical counts are collected properly.DEBUG_ONLY(current->reset_visited_for_critical_count(active_safepoint_counter);)ThreadSafepointState* cur_state = current->safepoint_state();assert(!cur_state->is_running(), "Thread not suspended at safepoint");cur_state->restart(); // TSS _runningassert(cur_state->is_running(), "safepoint state has not been reset");}} // ~JavaThreadIteratorWithHandle// Release threads lock, so threads can be created/destroyed again.Threads_lock->unlock();// Wake threads after local state is correctly set._wait_barrier->disarm();
}// Guarantees any thread that called wait() will be awake when it returns.// Provides a trailing fence.void disarm() {assert(_owner == Thread::current(), "Not owner thread");_impl.disarm();}// Guarantees any thread that called wait() will be awake when it returns.// Provides a trailing fence.void disarm() {assert(_owner == Thread::current(), "Not owner thread");_impl.disarm();}void LinuxWaitBarrier::disarm() {assert(_futex_barrier != 0, "Should be armed/non-zero.");_futex_barrier = 0;int s = futex(&_futex_barrier,FUTEX_WAKE_PRIVATE,INT_MAX /* wake a max of this many threads */);guarantee_with_errno(s > -1, "futex FUTEX_WAKE failed");
}static int futex(volatile int *addr, int futex_op, int op_arg) {return syscall(SYS_futex, addr, futex_op, op_arg, NULL, NULL, 0);
}