openMPM源码分析（三）

柔光的暖阳◎ 2022-08-06 12:22 251阅读 0赞

**函数 int32\_t sync\_test\_lock(void)**

\- 一、主要涉及两个函数mpm\_sync\_lock\_acquire(void\*lock,int32\_t user\_id)和mpm\_sync\_lock\_release(void\* lock, int32\_t user\_id)顾名思义就是锁的获得和锁的释放。

\- 二、函数 int32\_t mpm\_sync\_lock\_acquire(void \*lock,int32\_t user\_id );

\- 源码

int32_t mpm_sync_lock_acquire(void *lock, int32_t user_id)
    {
      mpm_sync_lock_t     *inst   = (mpm_sync_lock_t *)lock;
      mpm_sync_lock_user_t *users  = &inst->users[0];
      int32_t         i, ret = MPM_SYNC_LOCK_ERROR_NONE;
    
      if ( inst == NULL )
        ret = MPM_SYNC_LOCK_ERROR_HANDLE;
      else if ( ( user_id < 0) || (user_id >= inst->num_users) )
        ret = MPM_SYNC_LOCK_ERROR_USER_ID;
      else if ( ( users[user_id].number > 0 ) || (users[user_id].entering == 1))
        ret = MPM_SYNC_LOCK_ERROR_DBL_LOCK;
      else 
      {
        /* Take a number */
        users[user_id].entering = 1;
    
        /* number[i] = 1 + max(number[1:N]) */
        users[user_id].number = 0;
        for ( i = 0; i < inst->num_users; i++)
        {
          if ( users[user_id].number < users[i].number )
          {
            users[user_id].number = users[i].number;
          }
        }
        users[user_id].number++;
    
        users[user_id].entering = 0;
    
        for ( i = 0; i < inst->num_users; i++ )
        {
          /* Wait for the other guy to get a number */
          while ( users[i].entering);
          while ( ( users[i].number != 0) &&                           // This guy is waiting...
                  ( ( users[i].number < users[user_id].number ) ||     // They have a better number...
                    ( ( users[i].number == users[user_id].number ) &&  // They have the same number...
                      ( i < user_id )                                  // They are more important than you.
                    )
                  )
                );
        }
      }
      return ( ret );
    }

\- 数据结构mpm\_sync\_lock\_user\_t & mpm\_sync\_lock\_t

typedef struct {
      volatile int32_t entering;
      volatile int32_t number;
    } mpm_sync_lock_user_t;
    
    typedef struct {
      int32_t                num_users;
      mpm_sync_lock_user_t   users[1];
    } mpm_sync_lock_t;

\- 在本函数所用的面包店算法简介：顾客（进程）进入面包店（临界区）之前需要排队领号，号码从小到大一次发放。假如我现在去面包店（当前进程），那么我首先抽号，再依次与门口所有的人比较，如果我票号最小，我直接进去，如果你比我小，等你进去面包店，我再与剩下的所有人比较。参考：http://zh.wikipedia.org/zh-cn/Lamport%E9%9D%A2%E5%8C%85%E5%BA%97%E7%AE%97%E6%B3%95

\- 知道了面包店算法就不难理解函数代码：

\- 从数据结构说起，mpm\_sync\_lock\_user\_t这个结构体里面的成员变量entering\{true,false\}和number\{从小到大\}分别表示core\[i\]正在获取它的排队号和core\[i\]的当前排队登记号。

\- 遵循面包店算法，拿号，如下图所示：

![SouthEast][]

\- 遵循面包店算法，遵循规则\{(a,b)<(c,d)\}进行比较，当两个core同时访问lock内存区的核具有相同的number时，coreID较小的具有较高的优先级。，如下图所示：

![SouthEast 1][]

\-三、 函数 int32\_t mpm\_sync\_lock\_release(void \*lock,int32\_t user\_id );

\- 源码

int32_t mpm_sync_lock_release(void *lock, int32_t user_id)
    {
      mpm_sync_lock_t      *inst   = (mpm_sync_lock_t *)lock;
      mpm_sync_lock_user_t  *users  = &inst->users[0];
      int32_t          ret    = MPM_SYNC_LOCK_ERROR_NONE;
    
      if ( inst == NULL )
        ret = MPM_SYNC_LOCK_ERROR_HANDLE;
      else if ( ( user_id < 0) || (user_id >= inst->num_users) )
        ret = MPM_SYNC_LOCK_ERROR_USER_ID;
      else if ( users[user_id].number == 0 )
        ret = MPM_SYNC_LOCK_ERROR_DBL_UNLOCK;
      else
        users[user_id].number = 0;
    
      return ( ret );
    }

\-四、 面包店算法伪代码

// declaration and initial values of global variables
         Entering: array [1..NUM_THREADS] of bool = {false};
         Number: array [1..NUM_THREADS] of integer = {0};
     
       lock(integer i) {
            Entering[i] = true;
            Number[i] = 1 + max(Number[1], ..., Number[NUM_THREADS]);
            Entering[i] = false;
            for (j = 1; j <= NUM_THREADS; j++) {
                // Wait until thread j receives its number:
                while (Entering[j]) { /* nothing */ }
                // Wait until all threads with smaller numbers or with the same
                // number, but with higher priority, finish their work:
               while ((Number[j] != 0) && ((Number[j], j) < (Number[i], i))) { /* nothing */ }
           }
       }
       
       unlock(integer i) {
           Number[i] = 0;
       }
     
       Thread(integer i) {
           while (true) {
               lock(i);//对应mpm_sync_lock_acquire()
               // The critical section goes here...
               unlock(i);//对应mpm_sync_lock_release()
               // non-critical section...
           }
       }

[SouthEast]: /images/20220806/473e7e2978054d2ab6a308f95a8daf5e.png
[SouthEast 1]: /images/20220806/db19f06a5b9e4e998723f11152d8ce00.png