MySQL · 引擎特性 · Latch 持有分析

MySQL · 引擎特性 · Latch 持有分析

Introduction

mysql中latch没有死锁检测机制,通常指的是server层、innodb层的互斥锁和读写锁。当出现问题后,需要从现场core文件排查,下面介绍如何排查锁被谁持有了

Mutex in Server

除了win之外都采用了glibc中的pthread_mutex_t,如server层中LOCK_status, LOCK_thd_remove

方法一:

(gdb) p LOCK_status
$11 = {m_mutex = {__data = {__lock = 2, __count = 0, __owner = 102188, __nusers = 1, __kind = 3, __spins = 85, __list = {__prev = 0x0, __next = 0x0}},
    __size = "\002\000\000\000\000\000\000\000,\217\001\000\001\000\000\000\003\000\000\000U", '\000' <repeats 18 times>, __align = 2}, m_psi = 0x0}

这里的__owner为core中LWP XXXX后的值

MySQL · 引擎特性 · Latch 持有分析

方法二:

切换到__lll_lock_wait这样frame上,对于64 bit系统:

(gdb) p *(pthread_mutex_t*)$rdi
$12 = {__data = {__lock = 2, __count = 0, __owner = 102188, __nusers = 1, __kind = 3, __spins = 85, __list = {__prev = 0x0, __next = 0x0}},
  __size = "\002\000\000\000\000\000\000\000,\217\001\000\001\000\000\000\003\000\000\000U", '\000' <repeats 18 times>, __align = 2}

同样能找到pthread_mutex中的owner

RW_lock in server

除了win之外都采用了glibc中的pthread_rwlock_t

(gdb) frame 1
#1  0x0000000000ec2059 in native_rw_wrlock (rwp=0x7f5faf078298) at /home/admin/129_20200113173827294_121311408_code/rpm_workspace/include/thr_rwlock.h:101
101     /home/admin/129_20200113173827294_121311408_code/rpm_workspace/include/thr_rwlock.h: No such file or directory.
(gdb) p rwp
$13 = (native_rw_lock_t *) 0x7f5faf078298
(gdb) p *rwp
$14 = {__data = {__lock = 0, __nr_readers = 0, __readers_wakeup = 0, __writer_wakeup = 0, __nr_readers_queued = 0, __nr_writers_queued = 15, __writer = 61789, __shared = 0, __pad1 = 0, __pad2 = 0, __flags = 0},
  __size = '\000' <repeats 20 times>, "\017\000\000\000]\361", '\000' <repeats 29 times>, __align = 0}
  • __nr_readers: 当前有多少个线程持有读锁
  • __nr_readers_queued: 当前有多少个线程在等待获得读锁
  • __nr_writers_queued: 当前有多少个线程在等待获得写锁,PS:写锁的优先级比读锁要高。即如果线程想获得读锁,当发现__nr_writers_queued不为0时,哪怕当前没有人获得写锁,也会将自己阻塞。目的是防止写锁饿死。
  • __writer:写锁持有者的LWP #

如果有线程持有写锁,通过__writer很容易找到该线程;如果有线程持有了读锁,持有读锁的线程和位置可能有多个,则可以尝试通过下述方法进行排查:

$ gdb <binary> <coredump> -ex "thread apply all bt" -ex "quit" > core.bt
$ pt-pmp core.bt > pt-pmp.log

pt-pmp.log中,排除:

  1. 出现频次高于__nr_readers的堆栈
  2. 阻塞在获取该锁的写锁的所有线程
  3. 带有poll()epoll_wait的堆栈
  4. 带有pthread_cond_wait的堆栈持有该读锁的可能性也比较低

由于持有读锁的线程和位置可能有多个,排查读锁持有者需要根据具体情况分析。

RW_lock in Innodb

innodb层的读写锁,如dict_operation_lockbtr_search_latchescheckpoint_lock

(gdb) p *dict_operation_lock
$16 = {lock_word = -2, waiters = 1, recursive = true, sx_recursive = 0, writer_is_wait_ex = false, writer_thread = 140042102085376, event = 0x7f5faf05aab8, wait_ex_event = 0x7f5faf05ab58,
  cfile_name = 0x162c6d8 "/home/admin/129_20200113173827294_121311408_code/rpm_workspace/storage/innobase/dict/dict0dict.cc",
  last_s_file_name = 0x1619240 "/home/admin/129_20200113173827294_121311408_code/rpm_workspace/storage/innobase/row/row0undo.cc",
  last_x_file_name = 0x1614968 "/home/admin/129_20200113173827294_121311408_code/rpm_workspace/storage/innobase/row/row0mysql.cc", cline = 1186, is_block_lock = 0, last_s_line = 322, last_x_line = 4290, count_os_wait = 20559,
  list = {prev = 0x7f5faea79150, next = 0x7f5faea87428}, pfs_psi = 0x0}
  • lock_word = X_LOCK_DECR时,意味着当前锁没有被任何人持有
  • X_LOCK_HALF_DECR < lock_word < X_LOCK_DECR,意味着当前有一个或多个线程持有读锁
  • 0 < lock_word <= X_LOCK_HALF_DECR时,意味着当前有一个线程持有SX锁,有0个(lock_word = X_LOCK_HALF_DECR)或多个线程(lock_word < X_LOCK_HALF_DECR)持有读锁
  • lock_word = 0时表示没有线程持有读锁,下一个写锁已经加上(并已获得)
  • lock_word < 0是表示有线程持有一个或多个读锁,下一个写锁已经预定(仍未获得,在等待读锁释放)
  1. 这里SX锁是一种介于X锁和S锁的锁,它阻塞XSX锁,但不阻塞S
  2. 为了更好理解lock_word的含义,下面简单介绍rw_lock_t获取写锁的操作
// lock_word 的初始值,意味着最多允许0x20000000个读锁同时持有
#define X_LOCK_DECR     0x20000000
// 当上SX锁时,会尝试将lock_word减少X_LOCK_HALF_DECR
#define X_LOCK_HALF_DECR    0x10000000

rw_lock_x_lock_low(rw_lock_t*  lock, ulint pass, const char* file_name, ulint line) {

  // 如果lock_word>X_LOCK_HALF_DECR,尝试将lock_word减少X_LOCK_DECR
  // 如果成功,则至少预定自己为下一个写锁的持有者,返回true,否则返回false
  if (rw_lock_lock_word_decr(lock, X_LOCK_DECR, X_LOCK_HALF_DECR)) {
  
    // 预定自己为下一个写锁持有者,此时lock_word<=0,last_x_file_name:last_x_line 为上一个写锁持有者的上锁位置
    // 将自己的线程标识写入writer_thread,
    rw_lock_set_writer_id_and_recursion_flag(lock, !pass);)

    // 如果lock_word<0,说明有线程持有读锁,必须等待读锁释放
    // 阻塞直到 lock_word==0, 
    rw_lock_x_lock_wait(lock, pass, 0, file_name, line);

  } else {
    ......
  }
  
  // 成功获得写锁,last_x_file_name:last_x_line指向加锁的位置
  lock->last_x_file_name = file_name;
  lock->last_x_line = (unsigned int) line;

  return true;
}

再回到上述的例子:

  • lock_word=-2,说明这里有两个线程持有了读锁,从last_s_file_name : last_s_line 可以看到加读锁的位置;
  • 同时,下一个写锁已经预定,预定者由writer_thread指明;
  • 但是,last_x_file_name : last_x_line 并不是预订者的位置,因为此时写锁还没有真正持有
  • writer_thread指明了持有或即将持有写锁的线程id,将其转成16进制可以在堆栈中搜出:

MySQL · 引擎特性 · Latch 持有分析

另外:

  • 如果拿不到锁,线程会尝试自旋一段时间,如果自旋后还是拿不到锁,则让出处理器
  • 自旋的时间由innodb参数innodb_sync_spin_loopsinnodb_spin_wait_delay决定
  • 如果发现所有的拿锁的线程都处于自旋状态,则可以尝试减少innodb_sync_spin_loopsinnodb_spin_wait_delay

Mutex in Innodb

innodb层最常见的mutex latchPolicyMutex<TTASEventMutex<GenericPolicy>,这种锁和rw_lock_t一样是spin锁,当拿不到锁时会尝试自旋一段时间:

spin_and_try_lock(...)
{
  ...
  for (;;) {
    // 尝试自旋,自旋的时间同样由由`innodb_sync_spin_loops`、`innodb_spin_wait_delay`决定
    is_free(max_spins, max_delay, n_spins) {
      if (try_lock()) {
        break;
      } else {
        ...
      }
    } else {
      max_spins = n_spins + step;
    }
    os_thread_yield();
    ...
  }
  ...
}

这种锁一般持有时间很短,在innodb上采用atomic来实现,目前没有好的办法排查加这种锁的线程和位置,但是core文件仍然提供了许多有用的信息:

(gdb) p *this
$19 = {m_impl = {m_lock_word = 0, m_waiters = 0, m_event = 0x7f5faea51358, m_policy = {m_count = {m_spins = 0, m_waits = 0, m_calls = 0, m_enabled = false}, m_id = LATCH_ID_FLUSH_LIST}}, m_ptr = 0x0}

m_lock_word对应值的含义:

/** Mutex is free */
 MUTEX_STATE_UNLOCKED = 0
 
 /** Mutex is acquired by some thread. */
 MUTEX_STATE_LOCKED = 1
 
 /** Mutex is contended and there are threads waiting on the lock. */
 MUTEX_STATE_WAITERS = 2

另外m_waiters = 0并不意味着目前没有等锁的线程,如果拿该锁的线程都处于自旋状态,m_waiters仍然等于0

如果有线程持有该锁,想要排查,同样可以用pt-pmp排查:

  1. 排除堆栈重复次数超过1次的所有线程
  2. 排除阻塞在获取该锁的所有线程
  3. 排除带有poll()epoll_wait的堆栈
  4. 带有pthread_cond_wait的堆栈持有该锁的可能性也比较低
  5. 阻塞在__lll_lock_wait的线程持有该锁的可能性比较低,持有innodb层mutex锁的线程阻塞在server层锁的可能性比较低

持有该锁的堆栈只可能出现1次,排查持有者需要根据具体情况分析

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