jvm安全点(二)openjdk17 c++源码垃圾回收安全点信号函数处理线程阻塞
1. 信号处理与桩代码（Stub）
当线程访问安全点轮询页（Polling Page）时：
- 触发 SIGSEGV 信号：访问只读的轮询页会引发
SIGSEGV异常。 - 信号处理函数：
pd_hotspot_signal_handler检测到SafepointMechanism::is_poll_address为真，调用SharedRuntime::get_poll_stub获取桩代码入口地址（如polling_page_safepoint_handler_blob）。 - 篡改 PC：
os::Posix::ucontext_set_pc(uc, stub)将线程的 程序计数器（PC） 设置为桩代码地址。
2. 桩代码的职责
桩代码（如 polling_page_safepoint_handler_blob）是平台相关的汇编代码，其核心逻辑为：
asm
复制
// 伪代码示例 call SafepointSynchronize::handle_polling_page_exception ; 调用安全点处理函数 ret
- 直接调用：桩代码通过
call指令直接调用SafepointSynchronize::handle_polling_page_exception。 - 触发阻塞：
handle_polling_page_exception最终通过SafepointSynchronize::block让线程阻塞在安全点。
3. 关键调用链
信号处理与安全点处理的完整路径：
信号处理函数 (javaSignalHandler)→ PosixSignals::pd_hotspot_signal_handler→ 检测到安全点轮询页（SafepointMechanism::is_poll_address）→ SharedRuntime::get_poll_stub(pc) 获取桩代码地址→ 篡改 PC 到桩代码（如 polling_page_safepoint_handler_blob）→ 桩代码执行→ SafepointSynchronize::handle_polling_page_exception→ SafepointMechanism::process→ SafepointSynchronize::block→ 线程阻塞等待安全点4. 核心设计思想
- 信号驱动：通过操作系统的内存保护机制（轮询页不可访问）触发信号，将控制权交给 JVM。
- 间接跳转：信号处理函数不直接调用安全点逻辑，而是通过修改线程执行路径（PC），跳转到桩代码。
- 桩代码桥接：桩代码作为 桥梁，将信号处理上下文与 JVM 内部安全点处理逻辑连接。
5. 普通线程阻塞的触发
- 所有 Java 线程：无论是用户线程、JIT 编译代码线程，还是解释器执行的线程，访问轮询页时都会触发此流程。
- 统一入口：无论线程原本在执行什么，最终都会通过桩代码调用
handle_polling_page_exception，确保所有线程在安全点处阻塞。
总结
- 信号处理函数不直接调用：
handle_polling_page_exception由 桩代码 直接调用，而非信号处理函数本身。 - 间接触发阻塞：通过篡改 PC 到桩代码，再由桩代码触发安全点处理逻辑，最终实现线程阻塞。
- 统一安全点处理：所有 Java 线程通过此机制在安全点同步，确保 GC 等操作的安全执行。
##源码
address SharedRuntime::get_poll_stub(address pc) {address stub;// Look up the code blobCodeBlob *cb = CodeCache::find_blob(pc);// Should be an nmethodguarantee(cb != NULL && cb->is_compiled(), "safepoint polling: pc must refer to an nmethod");// Look up the relocation informationassert(((CompiledMethod*)cb)->is_at_poll_or_poll_return(pc),"safepoint polling: type must be poll");#ifdef ASSERTif (!((NativeInstruction*)pc)->is_safepoint_poll()) {tty->print_cr("bad pc: " PTR_FORMAT, p2i(pc));Disassembler::decode(cb);fatal("Only polling locations are used for safepoint");}
#endifbool at_poll_return = ((CompiledMethod*)cb)->is_at_poll_return(pc);bool has_wide_vectors = ((CompiledMethod*)cb)->has_wide_vectors();if (at_poll_return) {assert(SharedRuntime::polling_page_return_handler_blob() != NULL,"polling page return stub not created yet");stub = SharedRuntime::polling_page_return_handler_blob()->entry_point();} else if (has_wide_vectors) {assert(SharedRuntime::polling_page_vectors_safepoint_handler_blob() != NULL,"polling page vectors safepoint stub not created yet");stub = SharedRuntime::polling_page_vectors_safepoint_handler_blob()->entry_point();} else {assert(SharedRuntime::polling_page_safepoint_handler_blob() != NULL,"polling page safepoint stub not created yet");stub = SharedRuntime::polling_page_safepoint_handler_blob()->entry_point();}log_debug(safepoint)("... found polling page %s exception at pc = "INTPTR_FORMAT ", stub =" INTPTR_FORMAT,at_poll_return ? "return" : "loop",(intptr_t)pc, (intptr_t)stub);return stub;
}bool PosixSignals::pd_hotspot_signal_handler(int sig, siginfo_t* info,ucontext_t* uc, JavaThread* thread) {if (sig == SIGILL &&((info->si_addr == (caddr_t)check_simd_fault_instr)|| info->si_addr == (caddr_t)check_vfp_fault_instr|| info->si_addr == (caddr_t)check_vfp3_32_fault_instr|| info->si_addr == (caddr_t)check_mp_ext_fault_instr)) {// skip faulty instruction + instruction that sets return value to// success and set return value to failure.os::Posix::ucontext_set_pc(uc, (address)info->si_addr + 8);uc->uc_mcontext.arm_r0 = 0;return true;}address stub = NULL;address pc = NULL;bool unsafe_access = false;if (info != NULL && uc != NULL && thread != NULL) {pc = (address) os::Posix::ucontext_get_pc(uc);// Handle ALL stack overflow variations hereif (sig == SIGSEGV) {address addr = (address) info->si_addr;// check if fault address is within thread stackif (thread->is_in_full_stack(addr)) {// stack overflowStackOverflow* overflow_state = thread->stack_overflow_state();if (overflow_state->in_stack_yellow_reserved_zone(addr)) {overflow_state->disable_stack_yellow_reserved_zone();if (thread->thread_state() == _thread_in_Java) {// Throw a stack overflow exception. Guard pages will be reenabled// while unwinding the stack.stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::STACK_OVERFLOW);} else {// Thread was in the vm or native code. Return and try to finish.return true;}} else if (overflow_state->in_stack_red_zone(addr)) {// Fatal red zone violation. Disable the guard pages and fall through// to handle_unexpected_exception way down below.overflow_state->disable_stack_red_zone();tty->print_raw_cr("An irrecoverable stack overflow has occurred.");} else {// Accessing stack address below sp may cause SEGV if current// thread has MAP_GROWSDOWN stack. This should only happen when// current thread was created by user code with MAP_GROWSDOWN flag// and then attached to VM. See notes in os_linux.cpp.if (thread->osthread()->expanding_stack() == 0) {thread->osthread()->set_expanding_stack();if (os::Linux::manually_expand_stack(thread, addr)) {thread->osthread()->clear_expanding_stack();return true;}thread->osthread()->clear_expanding_stack();} else {fatal("recursive segv. expanding stack.");}}}}if (thread->thread_state() == _thread_in_Java) {// Java thread running in Java code => find exception handler if any// a fault inside compiled code, the interpreter, or a stubif (sig == SIGSEGV && SafepointMechanism::is_poll_address((address)info->si_addr)) {stub = SharedRuntime::get_poll_stub(pc);} else if (sig == SIGBUS) {// BugId 4454115: A read from a MappedByteBuffer can fault// here if the underlying file has been truncated.// Do not crash the VM in such a case.CodeBlob* cb = CodeCache::find_blob_unsafe(pc);CompiledMethod* nm = (cb != NULL) ? cb->as_compiled_method_or_null() : NULL;if ((nm != NULL && nm->has_unsafe_access()) || (thread->doing_unsafe_access() && UnsafeCopyMemory::contains_pc(pc))) {unsafe_access = true;}} else if (sig == SIGSEGV &&MacroAssembler::uses_implicit_null_check(info->si_addr)) {// Determination of interpreter/vtable stub/compiled code null exceptionCodeBlob* cb = CodeCache::find_blob_unsafe(pc);if (cb != NULL) {stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::IMPLICIT_NULL);}} else if (sig == SIGILL && *(int *)pc == NativeInstruction::zombie_illegal_instruction) {// Zombiestub = SharedRuntime::get_handle_wrong_method_stub();}} else if ((thread->thread_state() == _thread_in_vm ||thread->thread_state() == _thread_in_native) &&sig == SIGBUS && thread->doing_unsafe_access()) {unsafe_access = true;}// jni_fast_Get<Primitive>Field can trap at certain pc's if a GC kicks in// and the heap gets shrunk before the field access.if (sig == SIGSEGV || sig == SIGBUS) {address addr = JNI_FastGetField::find_slowcase_pc(pc);if (addr != (address)-1) {stub = addr;}}}if (unsafe_access && stub == NULL) {// it can be an unsafe access and we haven't found// any other suitable exception reason,// so assume it is an unsafe access.address next_pc = pc + Assembler::InstructionSize;if (UnsafeCopyMemory::contains_pc(pc)) {next_pc = UnsafeCopyMemory::page_error_continue_pc(pc);}
#ifdef __thumb__if (uc->uc_mcontext.arm_cpsr & PSR_T_BIT) {next_pc = (address)((intptr_t)next_pc | 0x1);}
#endifstub = SharedRuntime::handle_unsafe_access(thread, next_pc);}if (stub != NULL) {
#ifdef __thumb__if (uc->uc_mcontext.arm_cpsr & PSR_T_BIT) {intptr_t p = (intptr_t)pc | 0x1;pc = (address)p;// Clear Thumb mode bit if we're redirected into the ARM ISA based codeif (((intptr_t)stub & 0x1) == 0) {uc->uc_mcontext.arm_cpsr &= ~PSR_T_BIT;}} else {// No Thumb2 compiled stubs are triggered from ARM ISA compiled JIT'd code today.// The support needs to be added if that changesassert((((intptr_t)stub & 0x1) == 0), "can't return to Thumb code");}
#endif// save all thread context in case we need to restore itif (thread != NULL) thread->set_saved_exception_pc(pc);os::Posix::ucontext_set_pc(uc, stub);return true;}return false;
}