spin用户态加锁源码分析-白红宇

spin用户态加锁源码分析

阅读量：5130 次

发布时间：2019-06-13

本文共 15115 字，大约阅读时间需要 50 分钟。

源码分析版本：glic-2.9

spin_lock加锁

pthread_spin_lock：

intpthread_spin_lock (lock)     pthread_spinlock_t *lock;{  asm ("\n"       "1:\t" LOCK_PREFIX "decl %0\n\t" //锁总线，开始加锁，在%0       "jne 2f\n\t" //加锁不成功，jns 汇编指令检查 EFLAGS 寄存器的 SF(符号)位，如果为 0，说明 slock 原来的值为 1，则线程获得锁，然后跳到标签 2 的位置结束本次函数调用。如果 SF 位为 1，说明 slock 原来的值为 0 或负数，锁已被占用。       ".subsection 1\n\t"  //重新加锁       ".align 16\n"  //按照2的16次方对其，也就是8k       "2:\trep; nop\n\t"   //栈顶字单元出栈，返回         "cmpl $0, %0\n\t"  //做减法运算，探测锁是否可用          "jg 1b\n\t"  //锁可用，到1处加锁           "jmp 2b\n\t"  //跳转到2，           ".previous"            : "=m" (*lock)             : "m" (*lock));            return 0; }

所以可以看出，如果函数加锁成功了，那么就跳出，否则就会不停的去循环加锁。spin lock则可以理解为在一个while(1)循环中用内嵌的汇编代码实现的锁操作，一直在CPU上循环检测锁的状态。这样导致在线程时间片内线程空转。浪费了大量的系统资源，导致性能降低。

mutex：庞大的加锁过程.从实现原理上来讲，Mutex属于sleep-waiting类型的锁。例如在一个双核的机器上有两个线程(线程A和线程B)，它们分别运行在Core0和Core1上。假设线程A想要通过pthread_mutex_lock操作去得到一个临界区的锁，而此时这个锁正被线程B所持有，那么线程A就会被阻塞(blocking)，Core0 会在此时进行上下文切换(Context Switch)将线程A置于等待队列中，此时Core0就可以运行其他的任务(例如另一个线程C)而不必进行忙等待。而Spin lock则不然，它属于busy-waiting类型的锁，如果线程A是使用pthread_spin_lock操作去请求锁，那么线程A就会一直在 Core0上进行忙等待并不停的进行锁请求，直到得到这个锁为止。

int__pthread_mutex_lock (mutex)     pthread_mutex_t *mutex;{  assert (sizeof (mutex->__size) >= sizeof (mutex->__data));  int oldval; 获取线程id  pid_t id = THREAD_GETMEM (THREAD_SELF, tid);  int retval = 0;

互斥锁的类型：有以下几个取值空间：

　　PTHREAD_MUTEX_TIMED_NP，这是缺省值，也就是普通锁。当一个线程加锁以后，其余请求锁的线程将形成一个等待队列，并在解锁后按优先级获得锁。这种锁策略保证了资源分配的公平性。

　　PTHREAD_MUTEX_RECURSIVE_NP，嵌套锁，允许同一个线程对同一个锁成功获得多次，并通过多次unlock解锁。如果是不同线程请求，则在加锁线程解锁时重新竞争。

　　PTHREAD_MUTEX_ERRORCHECK_NP，检错锁，如果同一个线程请求同一个锁，则返回EDEADLK，否则与PTHREAD_MUTEX_TIMED_NP类型动作相同。这样就保证当不允许多次加锁时不会出现最简单情况下的死锁。

　　PTHREAD_MUTEX_ADAPTIVE_NP，适应锁，动作最简单的锁类型，仅等待解锁后重新竞争。

switch (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex),                PTHREAD_MUTEX_TIMED_NP))    {      /* Recursive mutex.  */    case PTHREAD_MUTEX_RECURSIVE_NP:      /* Check whether we already hold the mutex.  */      if (mutex->__data.__owner == id)    {      /* Just bump the counter.  */      if (__builtin_expect (mutex->__data.__count + 1 == 0, 0))        /* Overflow of the counter.  */        return EAGAIN;      ++mutex->__data.__count;      return 0;    }      /* We have to get the mutex.  */      LLL_MUTEX_LOCK (mutex);可以看到，这个兄弟成为了数据保护的最后一道防线了。只要通过这个检测，线程就算安全了，可以认为是偷渡的最后一道门槛，过了这个，就到米国了。这就涉及到了那个经典的“用户态原子操作”了，这个问题在 ulrich dreppler的文章中有论述，这个依赖于体系结构，例如386下的cmpx本身是多CPU原子操作，所以可以实现原子性操作，而对于PowerPC和MIPS这类RISC机型，人家也有自己的玩法，就是lwarx和swarx这个姐妹花，从而可以完成用户态原子操作

assert (mutex->__data.__owner == 0);      mutex->__data.__count = 1;      break;      /* Error checking mutex.  */    case PTHREAD_MUTEX_ERRORCHECK_NP:      /* Check whether we already hold the mutex.  */      if (__builtin_expect (mutex->__data.__owner == id, 0))    return EDEADLK;      /* FALLTHROUGH */    case PTHREAD_MUTEX_TIMED_NP:    simple:      /* Normal mutex.  */      LLL_MUTEX_LOCK (mutex);      assert (mutex->__data.__owner == 0);      break;    case PTHREAD_MUTEX_ADAPTIVE_NP:      if (! __is_smp)    goto simple;      if (LLL_MUTEX_TRYLOCK (mutex) != 0)    {      int cnt = 0;      int max_cnt = MIN (MAX_ADAPTIVE_COUNT,                 mutex->__data.__spins * 2 + 10);      do        {          if (cnt++ >= max_cnt)        {          LLL_MUTEX_LOCK (mutex);          break;        }#ifdef BUSY_WAIT_NOP          BUSY_WAIT_NOP;#endif        }      while (LLL_MUTEX_TRYLOCK (mutex) != 0);      mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;    }      assert (mutex->__data.__owner == 0);      break;    case PTHREAD_MUTEX_ROBUST_RECURSIVE_NP:    case PTHREAD_MUTEX_ROBUST_ERRORCHECK_NP:    case PTHREAD_MUTEX_ROBUST_NORMAL_NP:    case PTHREAD_MUTEX_ROBUST_ADAPTIVE_NP:      THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,             &mutex->__data.__list.__next);      oldval = mutex->__data.__lock;      do    {    again:      if ((oldval & FUTEX_OWNER_DIED) != 0)        {          /* The previous owner died.  Try locking the mutex.  */          int newval = id;#ifdef NO_INCR          newval |= FUTEX_WAITERS;#else          newval |= (oldval & FUTEX_WAITERS);#endif          newval        = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,                               newval, oldval);          if (newval != oldval)        {          oldval = newval;          goto again;        }          /* We got the mutex.  */          mutex->__data.__count = 1;          /* But it is inconsistent unless marked otherwise.  */          mutex->__data.__owner = PTHREAD_MUTEX_INCONSISTENT;          ENQUEUE_MUTEX (mutex);          THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);          /* Note that we deliberately exit here.  If we fall         through to the end of the function __nusers would be         incremented which is not correct because the old         owner has to be discounted.  If we are not supposed         to increment __nusers we actually have to decrement         it here.  */#ifdef NO_INCR          --mutex->__data.__nusers;#endif          return EOWNERDEAD;        }      /* Check whether we already hold the mutex.  */      if (__builtin_expect ((oldval & FUTEX_TID_MASK) == id, 0))        {          int kind = PTHREAD_MUTEX_TYPE (mutex);          if (kind == PTHREAD_MUTEX_ROBUST_ERRORCHECK_NP)        {          THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,                 NULL);          return EDEADLK;        }          if (kind == PTHREAD_MUTEX_ROBUST_RECURSIVE_NP)        {          THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,                 NULL);          /* Just bump the counter.  */          if (__builtin_expect (mutex->__data.__count + 1 == 0, 0))            /* Overflow of the counter.  */            return EAGAIN;          ++mutex->__data.__count;          return 0;        }        }      oldval = LLL_ROBUST_MUTEX_LOCK (mutex, id);      if (__builtin_expect (mutex->__data.__owner                == PTHREAD_MUTEX_NOTRECOVERABLE, 0))        {          /* This mutex is now not recoverable.  */          mutex->__data.__count = 0;          lll_unlock (mutex->__data.__lock,              PTHREAD_ROBUST_MUTEX_PSHARED (mutex));          THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);          return ENOTRECOVERABLE;        }    }      while ((oldval & FUTEX_OWNER_DIED) != 0);      mutex->__data.__count = 1;      ENQUEUE_MUTEX (mutex);      THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);      break;    case PTHREAD_MUTEX_PI_RECURSIVE_NP:    case PTHREAD_MUTEX_PI_ERRORCHECK_NP:    case PTHREAD_MUTEX_PI_NORMAL_NP:    case PTHREAD_MUTEX_PI_ADAPTIVE_NP:    case PTHREAD_MUTEX_PI_ROBUST_RECURSIVE_NP:    case PTHREAD_MUTEX_PI_ROBUST_ERRORCHECK_NP:    case PTHREAD_MUTEX_PI_ROBUST_NORMAL_NP:    case PTHREAD_MUTEX_PI_ROBUST_ADAPTIVE_NP:      {    int kind = mutex->__data.__kind & PTHREAD_MUTEX_KIND_MASK_NP;    int robust = mutex->__data.__kind & PTHREAD_MUTEX_ROBUST_NORMAL_NP;    if (robust)      /* Note: robust PI futexes are signaled by setting bit 0.  */      THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,             (void *) (((uintptr_t) &mutex->__data.__list.__next)                   | 1));    oldval = mutex->__data.__lock;    /* Check whether we already hold the mutex.  */    if (__builtin_expect ((oldval & FUTEX_TID_MASK) == id, 0))      {        if (kind == PTHREAD_MUTEX_ERRORCHECK_NP)          {        THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);        return EDEADLK;          }        if (kind == PTHREAD_MUTEX_RECURSIVE_NP)          {        THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);        /* Just bump the counter.  */        if (__builtin_expect (mutex->__data.__count + 1 == 0, 0))          /* Overflow of the counter.  */          return EAGAIN;        ++mutex->__data.__count;        return 0;          }      }    int newval = id;#ifdef NO_INCR    newval |= FUTEX_WAITERS;#endif    oldval = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,                              newval, 0);    if (oldval != 0)      {        /* The mutex is locked.  The kernel will now take care of           everything.  */        int private = (robust               ? PTHREAD_ROBUST_MUTEX_PSHARED (mutex)               : PTHREAD_MUTEX_PSHARED (mutex));        INTERNAL_SYSCALL_DECL (__err);        int e = INTERNAL_SYSCALL (futex, __err, 4, &mutex->__data.__lock,                      __lll_private_flag (FUTEX_LOCK_PI,                              private), 1, 0);        if (INTERNAL_SYSCALL_ERROR_P (e, __err)        && (INTERNAL_SYSCALL_ERRNO (e, __err) == ESRCH            || INTERNAL_SYSCALL_ERRNO (e, __err) == EDEADLK))          {        assert (INTERNAL_SYSCALL_ERRNO (e, __err) != EDEADLK            || (kind != PTHREAD_MUTEX_ERRORCHECK_NP                && kind != PTHREAD_MUTEX_RECURSIVE_NP));        /* ESRCH can happen only for non-robust PI mutexes where           the owner of the lock died.  */        assert (INTERNAL_SYSCALL_ERRNO (e, __err) != ESRCH || !robust);        /* Delay the thread indefinitely.  */        while (1)          pause_not_cancel ();          }        oldval = mutex->__data.__lock;        assert (robust || (oldval & FUTEX_OWNER_DIED) == 0);      }    if (__builtin_expect (oldval & FUTEX_OWNER_DIED, 0))      {        atomic_and (&mutex->__data.__lock, ~FUTEX_OWNER_DIED);        /* We got the mutex.  */        mutex->__data.__count = 1;        /* But it is inconsistent unless marked otherwise.  */        mutex->__data.__owner = PTHREAD_MUTEX_INCONSISTENT;        ENQUEUE_MUTEX_PI (mutex);        THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);        /* Note that we deliberately exit here.  If we fall           through to the end of the function __nusers would be           incremented which is not correct because the old owner           has to be discounted.  If we are not supposed to           increment __nusers we actually have to decrement it here.  */#ifdef NO_INCR        --mutex->__data.__nusers;#endif        return EOWNERDEAD;      }    if (robust        && __builtin_expect (mutex->__data.__owner                 == PTHREAD_MUTEX_NOTRECOVERABLE, 0))      {        /* This mutex is now not recoverable.  */        mutex->__data.__count = 0;        INTERNAL_SYSCALL_DECL (__err);        INTERNAL_SYSCALL (futex, __err, 4, &mutex->__data.__lock,                  __lll_private_flag (FUTEX_UNLOCK_PI,                          PTHREAD_ROBUST_MUTEX_PSHARED (mutex)),                  0, 0);        THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);        return ENOTRECOVERABLE;      }    mutex->__data.__count = 1;    if (robust)      {        ENQUEUE_MUTEX_PI (mutex);        THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);      }      }      break;    case PTHREAD_MUTEX_PP_RECURSIVE_NP:    case PTHREAD_MUTEX_PP_ERRORCHECK_NP:    case PTHREAD_MUTEX_PP_NORMAL_NP:    case PTHREAD_MUTEX_PP_ADAPTIVE_NP:      {    int kind = mutex->__data.__kind & PTHREAD_MUTEX_KIND_MASK_NP;    oldval = mutex->__data.__lock;    /* Check whether we already hold the mutex.  */    if (mutex->__data.__owner == id)      {        if (kind == PTHREAD_MUTEX_ERRORCHECK_NP)          return EDEADLK;        if (kind == PTHREAD_MUTEX_RECURSIVE_NP)          {        /* Just bump the counter.  */        if (__builtin_expect (mutex->__data.__count + 1 == 0, 0))          /* Overflow of the counter.  */          return EAGAIN;        ++mutex->__data.__count;        return 0;          }      }    int oldprio = -1, ceilval;    do      {        int ceiling = (oldval & PTHREAD_MUTEX_PRIO_CEILING_MASK)              >> PTHREAD_MUTEX_PRIO_CEILING_SHIFT;        if (__pthread_current_priority () > ceiling)          {        if (oldprio != -1)          __pthread_tpp_change_priority (oldprio, -1);        return EINVAL;          }        retval = __pthread_tpp_change_priority (oldprio, ceiling);        if (retval)          return retval;        ceilval = ceiling << PTHREAD_MUTEX_PRIO_CEILING_SHIFT;        oldprio = ceiling;        oldval          = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,#ifdef NO_INCR                             ceilval | 2,#else                             ceilval | 1,#endif                             ceilval);        if (oldval == ceilval)          break;        do          {        oldval          = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,                             ceilval | 2,                             ceilval | 1);        if ((oldval & PTHREAD_MUTEX_PRIO_CEILING_MASK) != ceilval)          break;        if (oldval != ceilval)          lll_futex_wait (&mutex->__data.__lock, ceilval | 2,                  PTHREAD_MUTEX_PSHARED (mutex));          }        while (atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,                            ceilval | 2, ceilval)           != ceilval);      }    while ((oldval & PTHREAD_MUTEX_PRIO_CEILING_MASK) != ceilval);    assert (mutex->__data.__owner == 0);    mutex->__data.__count = 1;      }      break;    default:      /* Correct code cannot set any other type.  */      return EINVAL;    }  /* Record the ownership.  */  mutex->__data.__owner = id;#ifndef NO_INCR  ++mutex->__data.__nusers;#endif  return retval;}

可以看到，里面最关键的函数式

LLL_MUTEX_LOCK (mutex);

而这个函数是一个宏定义

1 #ifndef LLL_MUTEX_LOCK 2 # define LLL_MUTEX_LOCK(mutex) \ 3 lll_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex)) 4 # define LLL_MUTEX_TRYLOCK(mutex) \ 5 lll_trylock ((mutex)->__data.__lock) 6 # define LLL_ROBUST_MUTEX_LOCK(mutex, id) \ 7 lll_robust_lock ((mutex)->__data.__lock, id, \ 8 PTHREAD_ROBUST_MUTEX_PSHARED (mutex)) 9 #endif10 11 看x86_64的实现12 这里就开始了一次线程切换，并给加上锁 13 #define lll_lock(futex, private) \14 15 (void) \16 ({ int ignore1, ignore2, ignore3;     \17 if (__builtin_constant_p (private) && (private) == LLL_PRIVATE)     \18 __asm __volatile (__lll_lock_asm_start     \19 ".subsection 1\n\t"     \20 ".type _L_lock_%=, @function\n"     \21 "_L_lock_%=:\n"     \22 "1:\tleaq %2, %%rdi\n"\目标地址传送指令： 将一个近地址指针写入到指定的寄存器。，将rdi里面的数据放到2中 23 "2:\tsubq $128, %%rsp\n" \ 应该是做减法运算 24 "3:\tcallq __lll_lock_wait_private\n" \ 25 "4:\taddq $128, %%rsp\n" \ 做加法运算 26 "5:\tjmp 24f\n" \ 跳转 27 "6:\t.size _L_lock_%=, 6b-1b\n\t" \ 28 ".previous\n" \ 29 LLL_STUB_UNWIND_INFO_5 \ 30 "24:" \ 31 : "=S" (ignore1), "=&D" (ignore2), "=m" (futex), \ 32 "=a" (ignore3) \ 33 : "0" (1), "m" (futex), "3" (0) \ 34 : "cx", "r11", "cc", "memory"); \ 35 else \ 36 __asm __volatile (__lll_lock_asm_start \ 37 ".subsection 1\n\t" \ 38 ".type _L_lock_%=, @function\n" \ 39 "_L_lock_%=:\n" \ 40 "1:\tleaq %2, %%rdi\n" \ 41 "2:\tsubq $128, %%rsp\n" \ 42 "3:\tcallq __lll_lock_wait\n" \ 43 "4:\taddq $128, %%rsp\n" \ 44 "5:\tjmp 24f\n" \ 45 "6:\t.size _L_lock_%=, 6b-1b\n\t" \ 46 ".previous\n" \ 47 LLL_STUB_UNWIND_INFO_5 \ 48 "24:" \ 49 : "=S" (ignore1), "=D" (ignore2), "=m" (futex), \ 50 "=a" (ignore3) \ 51 : "1" (1), "m" (futex), "3" (0), "0" (private) \ 52 : "cx", "r11", "cc", "memory"); \ 53 })

这个目前小弟还暂时不是很熟悉汇编，所以也只能分析到此了。具体里面哪个地方时切换和加锁还有待分析，先得去学习汇编了，希望有高手给指点一下这段代码。

转载于:https://www.cnblogs.com/gogly/archive/2012/09/27/2706269.html

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