java jdk 为什么一个线程空指针不退出
· 7 min read
背景
了解jdk异常的捕获原理
堆栈
(gdb) bt
#0 PosixSignals::pd_hotspot_signal_handler (sig=sig@entry=11, info=info@entry=0x7ffff7bfd330, uc=uc@entry=0x7ffff7bfd200, thread=0x7ffff00295a0) at /home/ubuntu/jdk/src/hotspot/os_cpu/linux_x86/os_linux_x86.cpp:201
#1 0x00007ffff7090f7d in JVM_handle_linux_signal (abort_if_unrecognized=1, ucVoid=0x7ffff7bfd200, info=0x7ffff7bfd330, sig=11) at /home/ubuntu/jdk/src/hotspot/os/posix/signals_posix.cpp:656
#2 JVM_handle_linux_signal (sig=11, info=0x7ffff7bfd330, ucVoid=0x7ffff7bfd200, abort_if_unrecognized=1) at /home/ubuntu/jdk/src/hotspot/os/posix/signals_posix.cpp:557
#3 <signal handler called>
#4 0x00007fffe8537640 in ?? ()
#5 0x0000000000000246 in ?? ()
#6 0x00007fffe8537734 in ?? ()
#7 0x00007ffff79f1858 in ?? () from /home/ubuntu/jdk/build/linux-x86_64-server-fastdebug/jdk/lib/server/libjvm.so
#8 0x00007ffff7bfe290 in ?? ()
#9 0x00007ffff734777a in VM_Version::get_processor_features () at /home/ubuntu/jdk/src/hotspot/cpu/x86/vm_version_x86.cpp:803
这里会返回true , 然后就跳过jdk的退出
bool PosixSignals::pd_hotspot_signal_handler(int sig, siginfo_t* info,
ucontext_t* uc, JavaThread* thread) {
/*
NOTE: does not seem to work on linux.
if (info == NULL || info->si_code <= 0 || info->si_code == SI_NOINFO) {
// can't decode this kind of signal
info = NULL;
} else {
assert(sig == info->si_signo, "bad siginfo");
}
*/
// decide if this trap can be handled by a stub
address stub = NULL;
address pc = NULL;
//%note os_trap_1
if (info != NULL && uc != NULL && thread != NULL) {
pc = (address) os::Posix::ucontext_get_pc(uc);
if (sig == SIGSEGV && info->si_addr == 0 && info->si_code == SI_KERNEL) {
// An irrecoverable SI_KERNEL SIGSEGV has occurred.
// It's likely caused by dereferencing an address larger than TASK_SIZE.
return false;
}
// Handle ALL stack overflow variations here
if (sig == SIGSEGV) {
address addr = (address) info->si_addr;
// check if fault address is within thread stack
if (thread->is_in_full_stack(addr)) {
// stack overflow
if (os::Posix::handle_stack_overflow(thread, addr, pc, uc, &stub)) {
return true; // continue
}
}
}
if ((sig == SIGSEGV) && VM_Version::is_cpuinfo_segv_addr(pc)) {
// Verify that OS save/restore AVX registers.
stub = VM_Version::cpuinfo_cont_addr();
}
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 stub
if (sig == SIGSEGV && SafepointMechanism::is_poll_address((address)info->si_addr)) {
stub = SharedRuntime::get_poll_stub(pc);
} else if (sig == SIGBUS /* && info->si_code == BUS_OBJERR */) {
// 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(pc);
CompiledMethod* nm = (cb != NULL) ? cb->as_compiled_method_or_null() : NULL;
bool is_unsafe_arraycopy = thread->doing_unsafe_access() && UnsafeCopyMemory::contains_pc(pc);
if ((nm != NULL && nm->has_unsafe_access()) || is_unsafe_arraycopy) {
address next_pc = Assembler::locate_next_instruction(pc);
if (is_unsafe_arraycopy) {
next_pc = UnsafeCopyMemory::page_error_continue_pc(pc);
}
stub = SharedRuntime::handle_unsafe_access(thread, next_pc);
}
}
else
#ifdef AMD64
if (sig == SIGFPE &&
(info->si_code == FPE_INTDIV || info->si_code == FPE_FLTDIV)) {
stub =
SharedRuntime::
continuation_for_implicit_exception(thread,
pc,
SharedRuntime::
IMPLICIT_DIVIDE_BY_ZERO);
#else
if (sig == SIGFPE /* && info->si_code == FPE_INTDIV */) {
// HACK: si_code does not work on linux 2.2.12-20!!!
int op = pc[0];
if (op == 0xDB) {
// FIST
// TODO: The encoding of D2I in x86_32.ad can cause an exception
// prior to the fist instruction if there was an invalid operation
// pending. We want to dismiss that exception. From the win_32
// side it also seems that if it really was the fist causing
// the exception that we do the d2i by hand with different
// rounding. Seems kind of weird.
// NOTE: that we take the exception at the NEXT floating point instruction.
assert(pc[0] == 0xDB, "not a FIST opcode");
assert(pc[1] == 0x14, "not a FIST opcode");
assert(pc[2] == 0x24, "not a FIST opcode");
return true;
} else if (op == 0xF7) {
// IDIV
stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::IMPLICIT_DIVIDE_BY_ZERO);
} else {
// TODO: handle more cases if we are using other x86 instructions
// that can generate SIGFPE signal on linux.
tty->print_cr("unknown opcode 0x%X with SIGFPE.", op);
fatal("please update this code.");
}
#endif // AMD64
} else if (sig == SIGSEGV &&
MacroAssembler::uses_implicit_null_check(info->si_addr)) {
// Determination of interpreter/vtable stub/compiled code null exception
stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::IMPLICIT_NULL);
}
} else if ((thread->thread_state() == _thread_in_vm ||
thread->thread_state() == _thread_in_native) &&
(sig == SIGBUS && /* info->si_code == BUS_OBJERR && */
thread->doing_unsafe_access())) {
address next_pc = Assembler::locate_next_instruction(pc);
if (UnsafeCopyMemory::contains_pc(pc)) {
next_pc = UnsafeCopyMemory::page_error_continue_pc(pc);
}
stub = SharedRuntime::handle_unsafe_access(thread, next_pc);
}
// 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;
}
}
}
#ifndef AMD64
// Execution protection violation
//
// This should be kept as the last step in the triage. We don't
// have a dedicated trap number for a no-execute fault, so be
// conservative and allow other handlers the first shot.
//
// Note: We don't test that info->si_code == SEGV_ACCERR here.
// this si_code is so generic that it is almost meaningless; and
// the si_code for this condition may change in the future.
// Furthermore, a false-positive should be harmless.
if (UnguardOnExecutionViolation > 0 &&
stub == NULL &&
(sig == SIGSEGV || sig == SIGBUS) &&
uc->uc_mcontext.gregs[REG_TRAPNO] == trap_page_fault) {
int page_size = os::vm_page_size();
address addr = (address) info->si_addr;
address pc = os::Posix::ucontext_get_pc(uc);
// Make sure the pc and the faulting address are sane.
//
// If an instruction spans a page boundary, and the page containing
// the beginning of the instruction is executable but the following
// page is not, the pc and the faulting address might be slightly
// different - we still want to unguard the 2nd page in this case.
//
// 15 bytes seems to be a (very) safe value for max instruction size.
bool pc_is_near_addr =
(pointer_delta((void*) addr, (void*) pc, sizeof(char)) < 15);
bool instr_spans_page_boundary =
(align_down((intptr_t) pc ^ (intptr_t) addr,
(intptr_t) page_size) > 0);
if (pc == addr || (pc_is_near_addr && instr_spans_page_boundary)) {
static volatile address last_addr =
(address) os::non_memory_address_word();
// In conservative mode, don't unguard unless the address is in the VM
if (addr != last_addr &&
(UnguardOnExecutionViolation > 1 || os::address_is_in_vm(addr))) {
// Set memory to RWX and retry
address page_start = align_down(addr, page_size);
bool res = os::protect_memory((char*) page_start, page_size,
os::MEM_PROT_RWX);
log_debug(os)("Execution protection violation "
"at " INTPTR_FORMAT
", unguarding " INTPTR_FORMAT ": %s, errno=%d", p2i(addr),
p2i(page_start), (res ? "success" : "failed"), errno);
stub = pc;
// Set last_addr so if we fault again at the same address, we don't end
// up in an endless loop.
//
// There are two potential complications here. Two threads trapping at
// the same address at the same time could cause one of the threads to
// think it already unguarded, and abort the VM. Likely very rare.
//
// The other race involves two threads alternately trapping at
// different addresses and failing to unguard the page, resulting in
// an endless loop. This condition is probably even more unlikely than
// the first.
//
// Although both cases could be avoided by using locks or thread local
// last_addr, these solutions are unnecessary complication: this
// handler is a best-effort safety net, not a complete solution. It is
// disabled by default and should only be used as a workaround in case
// we missed any no-execute-unsafe VM code.
last_addr = addr;
}
}
}
#endif // !AMD64
if (stub != NULL) {
// save all thread context in case we need to restore it
if (thread != NULL) thread->set_saved_exception_pc(pc);
os::Posix::ucontext_set_pc(uc, stub);
return true; ///////////////////////////////////////// 这里会是true
}
return false;
}