AliCloudDB MongoDB在开发过程中遇到一个无法正常退出的BUG,由于是Release版本的编译(-O3),debuginfo已经不能很好的展现出堆栈的情况。这时又该如何确定问题所在呢?本篇文章完整的记录了整个排查过程。 ## 场景 kill命令正常执行,但进程迟迟没有退出。非必现场景,在批量回收资源时比较容易出现,平时开发测试时没有遇到。从场景上看出现的概率并不高,可能是在某种极端条件下才能触发,由于第一次收到报告后没有保留现场,先到官方JIRA平台上去搜相关的BUG,无果,又盲目的尝试了几个场景后只能先Hold住,等待下次出现。 ## 排查 ### 确认BUG方向 很幸运,第二天BUG再次出现,僵尸进程?死循环?死锁?没有收到Kill信号?无数想法蹦出来,迅速登陆机器,查看现场,先从最简单的可能性开始开始排查。 `ps` `top`第一套组合拳,排除了僵尸进程可能性,并且TOP显示CPU使用率为0并不高;strace继续跟进查看,也没有发现有系统调用,最后在补一个pstack打印堆栈信息,全部线程都在wait。BUG的排查方向大致确定:线程间资源同步的问题(当然也不能排除是其他的可能性)。 ### 确认代码范围 详细分析pstack内容,从堆栈信息中看一个长相特别的(其他大部分的线程堆栈都是雷同的): ~~~ Thread 46 (Thread 0x2b5f079be700 (LWP 613)): #0 0x000000301800b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x0000000000fec763 in boost::thread::join() () #2 0x0000000000c8eebc in mongo::repl::ReplicationCoordinatorExternalStateImpl::shutdown() () #3 0x0000000000c93fed in mongo::repl::ReplicationCoordinatorImpl::shutdown() () #4 0x0000000000ad2463 in mongo::exitCleanly(mongo::ExitCode) () #5 0x0000000000f9c176 in ?? () #6 0x0000000000feb384 in ?? () #7 0x0000003018007851 in start_thread () from /lib64/libpthread.so.0 #8 0x000000300a4e767d in clone () from /lib64/libc.so.6 ~~~ 从函数名上看起来是MongoDB退出的关键路径,就从这里入手,人肉翻下源码: ~~~ 97 void ReplicationCoordinatorExternalStateImpl::shutdown() { 98 boost::lock_guard<boost::mutex> lk(_threadMutex); 99 if (_startedThreads) { 100 log() << "Stopping replication applier threads"; 101 _syncSourceFeedback.shutdown(); 102 _syncSourceFeedbackThread->join(); 103 _applierThread->join(); 104 BackgroundSync* bgsync = BackgroundSync::get(); 105 bgsync->shutdown(); 106 _producerThread->join(); 107 } 108 } ~~~ 这么多的join,到底是哪个呢。上GDB,我们来看看Thread 46到底在等谁。先加载symbol-file,失败,加载后堆栈变得更乱了,换`disassemble`命令,显示汇编信息: ~~~ 0x0000000000c8ee6b <+75>: mov %rsp,%rdi 0x0000000000c8ee6e <+78>: callq 0xdc8670 <_ZN5mongo6logger16LogstreamBuilder10makeStreamEv> 0x0000000000c8ee73 <+83>: mov 0x20(%rsp),%rdi 0x0000000000c8ee78 <+88>: lea 0x86e299(%rip),%rsi # 0x14fd118 0x0000000000c8ee7f <+95>: mov $0x24,%edx 0x0000000000c8ee84 <+100>: callq 0x1456550 <_ZSt16__ostream_insertIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_PKS3_l> 0x0000000000c8ee89 <+105>: mov %rsp,%rdi 0x0000000000c8ee8c <+108>: callq 0xdc88d0 <_ZN5mongo6logger16LogstreamBuilderD2Ev> 0x0000000000c8ee91 <+113>: lea 0x38(%rbp),%rdi 0x0000000000c8ee95 <+117>: callq 0xcce810 <_ZN5mongo4repl18SyncSourceFeedback8shutdownEv> 0x0000000000c8ee9a <+122>: mov 0xe8(%rbp),%rdi 0x0000000000c8eea1 <+129>: test %rdi,%rdi 0x0000000000c8eea4 <+132>: je 0xc8ef14 <_ZN5mongo4repl39ReplicationCoordinatorExternalStateImpl8shutdownEv+244> 0x0000000000c8eea6 <+134>: callq 0xfec600 <_ZN5boost6thread4joinEv> 0x0000000000c8eeab <+139>: mov 0xf0(%rbp),%rdi 0x0000000000c8eeb2 <+146>: test %rdi,%rdi 0x0000000000c8eeb5 <+149>: je 0xc8ef14 <_ZN5mongo4repl39ReplicationCoordinatorExternalStateImpl8shutdownEv+244> 0x0000000000c8eeb7 <+151>: callq 0xfec600 <_ZN5boost6thread4joinEv> => 0x0000000000c8eebc <+156>: callq 0xc284a0 <_ZN5mongo4repl14BackgroundSync3getEv> 0x0000000000c8eec1 <+161>: mov %rax,%rdi 0x0000000000c8eec4 <+164>: callq 0xc27f60 <_ZN5mongo4repl14BackgroundSync8shutdownEv> 0x0000000000c8eec9 <+169>: mov 0xf8(%rbp),%rdi 0x0000000000c8eed0 <+176>: test %rdi,%rdi 0x0000000000c8eed3 <+179>: je 0xc8ef14 <_ZN5mongo4repl39ReplicationCoordinatorExternalStateImpl8shutdownEv+244> 0x0000000000c8eed5 <+181>: callq 0xfec600 <_ZN5boost6thread4joinEv> 0x0000000000c8eeda <+186>: nopw 0x0(%rax,%rax,1) 0x0000000000c8eee0 <+192>: mov %rbx,%rdi 0x0000000000c8eee3 <+195>: callq 0x804a38 <pthread_mutex_unlock@plt> 0x0000000000c8eee8 <+200>: cmp $0x4,%eax 0x0000000000c8eeeb <+203>: je 0xc8eee0 <_ZN5mongo4repl39ReplicationCoordinatorExternalStateImpl8shutdownEv+192> 0x0000000000c8eeed <+205>: test %eax,%eax 0x0000000000c8eeef <+207>: jne 0xc8ef0f <_ZN5mongo4repl39ReplicationCoordinatorExternalStateImpl8shutdownEv+239> 0x0000000000c8eef1 <+209>: add $0x38,%rsp 0x0000000000c8eef5 <+213>: pop %rbx 0x0000000000c8eef6 <+214>: pop %rbp 0x0000000000c8eef7 <+215>: pop %r12 0x0000000000c8eef9 <+217>: pop %r13 0x0000000000c8eefb <+219>: retq ~~~ 查看`0x0000000000c8eeb7`地址的上下文,通过前后指令的函数符号名确定了目前代码是在`_applierThread->join()`,这里可以思考下是否还有的其他方法获得代码行。 `_applierThread` 同样也是个线程,如果shutdown在等它,那它又在等谁呢,回头继续查pstack输出,找到相关的堆栈: ~~~ Thread 34 (Thread 0x2b5f0a6d8700 (LWP 1355)): #0 0x000000301800b43c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x0000000000c2687b in mongo::repl::BackgroundSync::waitUntilPaused() () #2 0x0000000000cd192e in mongo::repl::SyncTail::tryPopAndWaitForMore(mongo::OperationContext*, mongo::repl::SyncTail::OpQueue*, mongo::repl::ReplicationCoordinator*) () #3 0x0000000000cd2823 in mongo::repl::SyncTail::oplogApplication() () #4 0x0000000000ccaaaf in mongo::repl::runSyncThread() () #5 0x0000000000feb384 in ?? () #6 0x0000003018007851 in start_thread () from /lib64/libpthread.so.0 #7 0x000000300a4e767d in clone () from /lib64/libc.so.6 ~~~ 注意这里与shutdown的等待是不同的,shutdown是在等待线程退出,而这里是在等待某个条件变量,再次上GDB,查看Thread 34 & backtrace 1, `info locals`: ~~~ _lastOpTimeFetched = {i = 4, secs = 1448986976}, _lastAppliedHash = 3485900536827542548, _lastFetchedHash = 3485900536827542548, _pause = false, _pausedCondition = {internal_mutex = { __data = {__lock = 0, __count = 0, __owner = 0, __nusers = 1, __kind = 0, __spins = 0, __list = {__prev = 0x0, __next = 0x0} }, __size = '\000' <repeats 12 times>, "\001", '\000' <repeats 26 times>, __align = 0 } ~~~ ### 找到代码BUG 看看代码怎么写的吧: ~~~ 469 void BackgroundSync::waitUntilPaused() { 470 boost::unique_lock<boost::mutex> lock(_mutex); 471 while (!_pause) { 472 _pausedCondition.wait(lock); 473 } 474 } ~~~ `_pause`变量一直都是0,所以会hang在这里。继续查看代码,跟踪`_pausedCondition`的调用,有两个函数会去唤醒,一个是stop,另一个是shutdown,但shutdown的调用应该在线程退后调用,以便释放资源。 同时,再次回过来在pstack中查看`BackgroundSync`的堆栈信息,想看看到底卡在了哪里。结果找不到`BackgroundSync`线程,问题更清晰了,所有`_pausedCondition`条件变量的唤醒,都是在该线程中完成的,如果`BackgroundSync`已经不存在了,一定会hang住。 再反复阅读代码,`BackgroundSync`在判断到`inShutdown()`时会自动结束生命周期,但结束后并没有更改`_pause`状态。 ## FIX BUG 解决办法是线程最后退出前执行stop函数,修改`_pause`状态,(shutdown会提前释放资源),查看官方最最新代码,确认有同样的修改,反向追踪提交,找到相关[JIRA](https://jira.mongodb.org/browse/SERVER-19782),抱怨JIRA的搜索弱爆了。 ## 复现和验证 为何该BUG出现的频率会非常低呢,主要在判断是否等待的条件上: ~~~ if (replCoord->isWaitingForApplierToDrain()) { 506 BackgroundSync::get()->waitUntilPaused(); 507 if (peek(&op)) { 508 // The producer generated a last batch of ops before pausing so return 509 // false so that we'll come back and apply them before signaling the drain 510 // is complete. 511 return false; 512 } 513 replCoord->signalDrainComplete(txn); 514 } ~~~ ~~~ 1908 case kActionWinElection: { 1909 boost::unique_lock<boost::mutex> lk(_mutex); 1910 _electionId = OID::gen(); 1911 _topCoord->processWinElection(_electionId, getNextGlobalOptime()); 1912 _isWaitingForDrainToComplete = true; 1913 const PostMemberStateUpdateAction nextAction = 1914 _updateMemberStateFromTopologyCoordinator_inlock(); 1915 invariant(nextAction != kActionWinElection); 1916 lk.unlock(); 1917 _performPostMemberStateUpdateAction(nextAction); 1918 break; 1919 } ~~~ 也就是说刚刚赢得了选举后会产生`_isWaitingForDrainToComplete == true`状态,恰巧这个时间窗口内进程接受到退出信号。复现的办法就是在`kActionWinElection`的后面加上sleep,以此来延长时间窗口,当然也可以通过GDB BLOCK的方式来复现。