spin用户态加锁源码分析

源码分析版本：glic-2.9

spin_lock加锁

pthread_spin_lock：

int
pthread_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

#ifndef LLL_MUTEX_LOCK
# define LLL_MUTEX_LOCK(mutex) \
lll_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex))
# define LLL_MUTEX_TRYLOCK(mutex) \
lll_trylock ((mutex)->__data.__lock)
# define LLL_ROBUST_MUTEX_LOCK(mutex, id) \
lll_robust_lock ((mutex)->__data.__lock, id, \
PTHREAD_ROBUST_MUTEX_PSHARED (mutex))
#endif

看x86_64的实现
这里就开始了一次线程切换，并给加上锁

#define lll_lock(futex, private) \

(void) \
({ int ignore1, ignore2, ignore3;     \
if (__builtin_constant_p (private) && (private) == LLL_PRIVATE)     \
__asm __volatile (__lll_lock_asm_start     \
".subsection 1\n\t"     \
".type _L_lock_%=, @function\n"     \
"_L_lock_%=:\n"     \
"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

"2:\tsubq $128, %%rsp\n" \ 应该是做减法运算

"3:\tcallq __lll_lock_wait_private\n" \

 25 "4:\taddq $128, %%rsp\n" \ 做加法运算

"5:\tjmp 24f\n" \ 跳转

"6:\t.size _L_lock_%=, 6b-1b\n\t" \
".previous\n" \ 

LLL_STUB_UNWIND_INFO_5 \ 
"24:" \ 

: "=S" (ignore1), "=&D" (ignore2), "=m" (futex), \ 

"=a" (ignore3) \ 

: "0" (1), "m" (futex), "3" (0) \ 

: "cx", "r11", "cc", "memory"); \ 

else \ 

__asm __volatile (__lll_lock_asm_start \ 

".subsection 1\n\t" \ 

".type _L_lock_%=, @function\n" \ 

"_L_lock_%=:\n" \ 

"1:\tleaq %2, %%rdi\n" \ 

"2:\tsubq $128, %%rsp\n" \ 

"3:\tcallq __lll_lock_wait\n" \ 

"4:\taddq $128, %%rsp\n" \ 

"5:\tjmp 24f\n" \ 

"6:\t.size _L_lock_%=, 6b-1b\n\t" \ 

".previous\n" \ 

LLL_STUB_UNWIND_INFO_5 \ 

"24:" \ 

: "=S" (ignore1), "=D" (ignore2), "=m" (futex), \ 

"=a" (ignore3) \ 

: "1" (1), "m" (futex), "3" (0), "0" (private) \ 

: "cx", "r11", "cc", "memory"); \ 

})

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。