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pthread_mutex_lock/unlock(1)

我们来考察下pthread中锁的实现。

首先看下初始化宏:PTHREAD_MUTEX_INITIALIZER。

# define PTHREAD_MUTEX_INITIALIZER   { { 0, 0, 0, 0, 0, __PTHREAD_SPINS, { 0, 0 } } }
/* Data structures for mutex handling.  The structure of the attribute
   type is not exposed on purpose.  */
/*删减了32位的代码*/
typedef union { struct __pthread_mutex_s { int __lock; unsigned int __count; int __owner; #ifdef __x86_64__ unsigned int __nusers; #endif /* KIND must stay at this position in the structure to maintain binary compatibility with static initializers. */ int __kind; #ifdef __x86_64__ short __spins; short __elision; __pthread_list_t __list; # define __PTHREAD_MUTEX_HAVE_PREV 1 /* Mutex __spins initializer used by PTHREAD_MUTEX_INITIALIZER. */ # define __PTHREAD_SPINS 0, 0 #else #endif } __data; char __size[__SIZEOF_PTHREAD_MUTEX_T]; long int __align; } pthread_mutex_t;
注意PTHREAD_MUTEX_INITIALIZER 是8个成员的结构体,与pthread_mutex_t定义相符。并且所有成为初始化为0。

初始化之后,我们接着看看pthread_mutex_lock操作:
  1 #ifndef __pthread_mutex_lock
  2 strong_alias (__pthread_mutex_lock, pthread_mutex_lock)
  3 hidden_def (__pthread_mutex_lock)
  4 #endif
  5 
  6 int
  7 __pthread_mutex_lock (pthread_mutex_t *mutex)
  8 {
  9   assert (sizeof (mutex->__size) >= sizeof (mutex->__data));
 10 
 11   unsigned int type = PTHREAD_MUTEX_TYPE_ELISION (mutex);
 12 
 13   LIBC_PROBE (mutex_entry, 1, mutex);
 14 
 15   if (__builtin_expect (type & ~(PTHREAD_MUTEX_KIND_MASK_NP
 16                  | PTHREAD_MUTEX_ELISION_FLAGS_NP), 0))
 17     return __pthread_mutex_lock_full (mutex);
 18 
 19   if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_NP))
 20     {
 21       FORCE_ELISION (mutex, goto elision);
 22     simple:
 23       /* Normal mutex.  */
 24       LLL_MUTEX_LOCK (mutex);
 25       assert (mutex->__data.__owner == 0);
 26     }
 27 #ifdef HAVE_ELISION
 28   else if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_ELISION_NP))
 29     {
 30   elision: __attribute__((unused))
 31       /* This case can never happen on a system without elision,
 32          as the mutex type initialization functions will not
 33      allow to set the elision flags.  */
 34       /* Don‘t record owner or users for elision case.  This is a
 35          tail call.  */
 36       return LLL_MUTEX_LOCK_ELISION (mutex);
 37     }
 38 #endif
 39   else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
 40                  == PTHREAD_MUTEX_RECURSIVE_NP, 1))
 41     {
 42       /* Recursive mutex.  */
 43       pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
 44 
 45       /* Check whether we already hold the mutex.  */
 46       if (mutex->__data.__owner == id)
 47     {
 48       /* Just bump the counter.  */
 49       if (__glibc_unlikely (mutex->__data.__count + 1 == 0))
 50         /* Overflow of the counter.  */
 51         return EAGAIN;
 52 
 53       ++mutex->__data.__count;
 54 
 55       return 0;
 56     }
 57 
 58       /* We have to get the mutex.  */
 59       LLL_MUTEX_LOCK (mutex);
 60 
 61       assert (mutex->__data.__owner == 0);
 62       mutex->__data.__count = 1;
 63     }
 64   else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
 65               == PTHREAD_MUTEX_ADAPTIVE_NP, 1))
 66     {
 67       if (! __is_smp)
 68     goto simple;
 69 
 70       if (LLL_MUTEX_TRYLOCK (mutex) != 0)
 71     {
 72       int cnt = 0;
 73       int max_cnt = MIN (MAX_ADAPTIVE_COUNT,
 74                  mutex->__data.__spins * 2 + 10);
 75       do
 76         {
 77           if (cnt++ >= max_cnt)
 78         {
 79           LLL_MUTEX_LOCK (mutex);
 80           break;
 81         }
 82           atomic_spin_nop ();
 83         }
 84       while (LLL_MUTEX_TRYLOCK (mutex) != 0);
 85 
 86       mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
 87     }
 88       assert (mutex->__data.__owner == 0);
 89     }
 90   else
 91     {
 92       pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
 93       assert (PTHREAD_MUTEX_TYPE (mutex) == PTHREAD_MUTEX_ERRORCHECK_NP);
 94       /* Check whether we already hold the mutex.  */
 95       if (__glibc_unlikely (mutex->__data.__owner == id))
 96     return EDEADLK;
 97       goto simple;
 98     }
 99 
100   pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
101 
102   /* Record the ownership.  */
103   mutex->__data.__owner = id;
104 #ifndef NO_INCR
105   ++mutex->__data.__nusers;
106 #endif
107 
108   LIBC_PROBE (mutex_acquired, 1, mutex);
109 
110   return 0;
111 }
首先看下第一句
assert (sizeof (mutex->__size) >= sizeof (mutex->__data));
这句的意思是成员_size和_data所占内存相同,我们来验证下。
char __size[__SIZEOF_PTHREAD_MUTEX_T]的字节数:40.
1 #ifdef __x86_64__
2 # if __WORDSIZE == 64
3 #  define __SIZEOF_PTHREAD_ATTR_T 56
4 #  define __SIZEOF_PTHREAD_MUTEX_T 40

另一方面_data中的字节数是int、short、unsigned、__pthread_list_t这些个加起来,刚好为40字节.

所以这个union在64位计算机上最大的空间为40个字节。

接着是:

1   unsigned int type = PTHREAD_MUTEX_TYPE_ELISION (mutex);
1 #define PTHREAD_MUTEX_TYPE_ELISION(m) 2   ((m)->__data.__kind & (127|PTHREAD_MUTEX_ELISION_NP))

因为__kind为0,所以这里的type显然为0;

1   if (__builtin_expect (type & ~(PTHREAD_MUTEX_KIND_MASK_NP
2                  | PTHREAD_MUTEX_ELISION_FLAGS_NP), 0))
3     return __pthread_mutex_lock_full (mutex);

这里的结果为0,所以显然不走这个分支。

 

PTHREAD_MUTEX_TIMED_NP值为0,所以我们的代码显然是进入如下第一行的分支。

根据注释/* Normal mutex. */,很可能是通过这里得到锁。我们继续探索下,

/* Mutex types.  */
enum
{
  PTHREAD_MUTEX_TIMED_NP,
  PTHREAD_MUTEX_RECURSIVE_NP,
  PTHREAD_MUTEX_ERRORCHECK_NP,
  PTHREAD_MUTEX_ADAPTIVE_NP
#if defined __USE_UNIX98 || defined __USE_XOPEN2K8
 1   if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_NP))
 2     {
 3       FORCE_ELISION (mutex, goto elision);
 4     simple:
 5       /* Normal mutex.  */
 6       LLL_MUTEX_LOCK (mutex);
 7       assert (mutex->__data.__owner == 0);
 8     }
 9 #ifdef HAVE_ELISION
10   else if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_ELISION_NP))
11     {
12   elision: __attribute__((unused))
13       /* This case can never happen on a system without elision,
14          as the mutex type initialization functions will not
15      allow to set the elision flags.  */
16       /* Don‘t record owner or users for elision case.  This is a
17          tail call.  */
18       return LLL_MUTEX_LOCK_ELISION (mutex);
19     }
20 #endif
21   else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
22                  == PTHREAD_MUTEX_RECURSIVE_NP, 1))
23     {
24       /* Recursive mutex.  */
25       pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
26 
27       /* Check whether we already hold the mutex.  */
28       if (mutex->__data.__owner == id)
29     {
30       /* Just bump the counter.  */
31       if (__glibc_unlikely (mutex->__data.__count + 1 == 0))
32         /* Overflow of the counter.  */
33         return EAGAIN;
34 
35       ++mutex->__data.__count;
36 
37       return 0;
38     }
39 
40       /* We have to get the mutex.  */
41       LLL_MUTEX_LOCK (mutex);
42 
43       assert (mutex->__data.__owner == 0);
44       mutex->__data.__count = 1;
45     }
46   else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
47               == PTHREAD_MUTEX_ADAPTIVE_NP, 1))
48     {
49       if (! __is_smp)
50     goto simple;
51 
52       if (LLL_MUTEX_TRYLOCK (mutex) != 0)
53     {
54       int cnt = 0;
55       int max_cnt = MIN (MAX_ADAPTIVE_COUNT,
56                  mutex->__data.__spins * 2 + 10);
57       do
58         {
59           if (cnt++ >= max_cnt)
60         {
61           LLL_MUTEX_LOCK (mutex);
62           break;
63         }
64           atomic_spin_nop ();
65         }
66       while (LLL_MUTEX_TRYLOCK (mutex) != 0);
67 
68       mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
69     }
70       assert (mutex->__data.__owner == 0);
71     }
72   else
73     {
74       pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
75       assert (PTHREAD_MUTEX_TYPE (mutex) == PTHREAD_MUTEX_ERRORCHECK_NP);
76       /* Check whether we already hold the mutex.  */
77       if (__glibc_unlikely (mutex->__data.__owner == id))
78     return EDEADLK;
79       goto simple;
80     }

这里的意思:将_data中的__lock作为参数填入lll_lock,注意,这里是宏定义。

#ifndef LLL_MUTEX_LOCK
# define LLL_MUTEX_LOCK(mutex)   lll_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex))
1 #if LLL_PRIVATE == 0 && LLL_SHARED == 128
2 # define PTHREAD_MUTEX_PSHARED(m) 3   ((m)->__data.__kind & 128)
4 #else

这里的PTHREAD_MUTEX_PSHARED将__kind字段和128做&操作,推测是第8个标志位用来标识该锁是否共享。

既然如此, 我们这里两者填入的都是0,但是第一个__lock在后续使用中有取地址的可能。

我们接着看看lll_lock:

1 #define lll_lock(futex, private)    2   __lll_lock (&(futex), private)

取了地址, 那么这里就是原mutex中__lock字段的地址和数值0.

 1 #define __lll_lock(futex, private)                                       2   ((void)                                                                3    ({                                                                    4      int *__futex = (futex);                                             5      if (__glibc_unlikely                                                6          (atomic_compare_and_exchange_bool_acq (__futex, 1, 0)))         7        {                                                                 8          if (__builtin_constant_p (private) && (private) == LLL_PRIVATE)  9            __lll_lock_wait_private (__futex);                           10          else                                                           11            __lll_lock_wait (__futex, private);                          12        }                                                                13    }))

根据值, 走__lll_lock_wait:

 1 /* Note that we need no lock prefix.  */
 2 #define atomic_exchange_acq(mem, newvalue)  3   ({ __typeof (*mem) result;                               4      if (sizeof (*mem) == 1)                               5        __asm __volatile ("xchgb %b0, %1"                       6              : "=q" (result), "=m" (*mem)                   7              : "0" (newvalue), "m" (*mem));                   8      else if (sizeof (*mem) == 2)                           9        __asm __volatile ("xchgw %w0, %1"                      10              : "=r" (result), "=m" (*mem)                  11              : "0" (newvalue), "m" (*mem));                  12      else if (sizeof (*mem) == 4)                          13        __asm __volatile ("xchgl %0, %1"                          14              : "=r" (result), "=m" (*mem)                  15              : "0" (newvalue), "m" (*mem));                  16      else                                      17        __asm __volatile ("xchgq %q0, %1"                      18              : "=r" (result), "=m" (*mem)                  19              : "0" ((atomic64_t) cast_to_integer (newvalue)),     20                "m" (*mem));                          21      result; })
 1 /* This function doesn‘t get included in libc.  */
 2 #if IS_IN (libpthread)
 3 void
 4 __lll_lock_wait (int *futex, int private)
 5 {
 6   if (*futex == 2)
 7     lll_futex_wait (futex, 2, private); /* Wait if *futex == 2.  */
 8 
 9   while (atomic_exchange_acq (futex, 2) != 0)
10     lll_futex_wait (futex, 2, private); /* Wait if *futex == 2.  */
11 }
12 #endif

所以到了关键的地方, 这里是将futex(&__lock)的值从0原子变为2就成功。否则调用lll_futex_wait,阻塞。这里的atomic_exchange_acq是一个返回旧值的原子操作,直接采用了内敛汇编(xchg)的方式,并且根据变量类型从而选取linux下不同的汇编指令。

到了这里,只要这个原子xchg的是正确的,并且阻塞与唤醒(wake up)之间的协议是正确的,那么这个mutex的语义就得到保证了。

 

我们接着看看lll_futex_wait是怎么样的(val = 2, private = 0):

1 /* Wait while *FUTEXP == VAL for an lll_futex_wake call on FUTEXP.  */
2 #define lll_futex_wait(futexp, val, private) 3   lll_futex_timed_wait (futexp, val, NULL, private)

参数多了个NULL(val = 2, timeout = NULL, private = 0),

1 #define lll_futex_timed_wait(futexp, val, timeout, private)     2   lll_futex_syscall (4, futexp,                                 3              __lll_private_flag (FUTEX_WAIT, private),  4              val, timeout)

展开__lll_private_flag

1 # else
2 #  define __lll_private_flag(fl, private) 3   ((fl) | THREAD_GETMEM (THREAD_SELF, header.private_futex))
4 # endif
1 # define THREAD_SELF 2   ({ struct pthread *__self;                              3      asm ("mov %%fs:%c1,%0" : "=r" (__self)                      4       : "i" (offsetof (struct pthread, header.self)));               5      __self;})

这里是从struct pthread中取得private_futex来计算的,值为0。这里实际上只保留了FUTEX_WAIT的值,同样为0.

 1 #define FUTEX_WAIT        0
 2 #define FUTEX_WAKE        1
 3 #define FUTEX_REQUEUE        3
 4 #define FUTEX_CMP_REQUEUE    4
 5 #define FUTEX_WAKE_OP        5
 6 #define FUTEX_OP_CLEAR_WAKE_IF_GT_ONE    ((4 << 24) | 1)
 7 #define FUTEX_LOCK_PI        6
 8 #define FUTEX_UNLOCK_PI        7
 9 #define FUTEX_TRYLOCK_PI    8
10 #define FUTEX_WAIT_BITSET    9
11 #define FUTEX_WAKE_BITSET    10
12 #define FUTEX_WAIT_REQUEUE_PI   11
13 #define FUTEX_CMP_REQUEUE_PI    12
14 #define FUTEX_PRIVATE_FLAG    128
15 #define FUTEX_CLOCK_REALTIME    256
16 
17 #define FUTEX_BITSET_MATCH_ANY    0xffffffff

所以这里的lll_futex_syscall调用简化为:

 lll_futex_syscall (4, futexp,   0,     2,   NULL)

我们接着看:

#define lll_futex_syscall(nargs, futexp, op, ...)                       \
  ({                                                                        INTERNAL_SYSCALL_DECL (__err);                                          long int __ret = INTERNAL_SYSCALL (futex, __err, nargs, futexp, op,                        __VA_ARGS__);                        (__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (__ret, __err))              ? -INTERNAL_SYSCALL_ERRNO (__ret, __err) : 0);                       })

这里的futex作为字符串字面量后续使用,__VA_ARGS__指代了2和NULL。

我们看一下INTERNAL_SYSCALL:

# define INTERNAL_SYSCALL(name, err, nr, args...)   INTERNAL_SYSCALL_NCS (__NR_##name, err, nr, ##args)

# define INTERNAL_SYSCALL_NCS(name, err, nr, args...)   ({                                              unsigned long int resultvar;                              LOAD_ARGS_##nr (args)                                  LOAD_REGS_##nr                                      asm volatile (                                      "syscall\n\t"                                      : "=a" (resultvar)                                      : "0" (name) ASM_ARGS_##nr : "memory", REGISTERS_CLOBBERED_BY_SYSCALL);       (long int) resultvar; })

那么这里的INTERNAL_SYSCALL_NCS调用, 参数为( __NR_futex,err,4, futexp,0, 2, NULL)。第四个参数开始为futexp,0, 2, NULL。

# define LOAD_ARGS_4(a1, a2, a3, a4)                         LOAD_ARGS_TYPES_4 (long int, a1, long int, a2, long int, a3,                        long int, a4)

# define LOAD_REGS_4                                 LOAD_REGS_TYPES_4 (long int, a1, long int, a2, long int, a3,                        long int, a4)

# define ASM_ARGS_4    ASM_ARGS_3, "r" (_a4)

 

将LOAD_ARGS_##nr (args)、LOAD_REGS_##nr、ASM_ARGS_##nr、REGISTERS_CLOBBERED_BY_SYSCALL展开带入,之后可将INTERNAL_SYSCALL_NCS转换为如下:

    unsigned long long int resultvar;
    long int __arg4 = (long int) (NULL);                               long int __arg3 = (long int) (2);                               long int __arg2 = (long int) (0);                               long int __arg1 = (long int) (futexp);                           \    
    register long int _a4 asm ("r10") = __arg4;                           register long int _a3 asm ("rdx") = __arg3;                           register long int _a2 asm ("rsi") = __arg2;                           register long int _a1 asm ("rdi") = __arg1;                           asm volatile (                        \          
    "syscall\n\t"                                      : "=a" (resultvar)                                      : "0" (__NR_futex), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4) : "memory", "cc", "r11", "cx");       (long long int) resultvar; })

这里的__NR_futex为找不到,这应该是个linux系统定义的系统调用号,并且由它来定义SYS_futex的值。

#define SYS_futex        __NR_futex
那么上面的那段代码真的确定是使用(FUTEX_WAIT)futex来陷入了阻塞吗? 
让我尝试将之前写的一段直接采用futex做同步区块的代码修改下做检验。
原代码:
 1 #include <stdio.h>
 2 #include <pthread.h>
 3 #include <linux/futex.h>
 4 #include <syscall.h>
 5 #include <unistd.h>
 6 #include <sys/time.h>
 7 
 8 
 9 #define NUM 1000
10 
11 
12 int num = 0;
13 int futex_addr = 0;
14 
15 int futex_wait(void* addr, int val){
16     return syscall(SYS_futex, addr, FUTEX_WAIT, val, NULL, NULL, 0);
17 }
18 int futex_wake(void* addr, int val){
19   return syscall(SYS_futex, addr, FUTEX_WAKE, val, NULL, NULL, 0);
20 }
21 
22 void* thread_f(void* par){
23         int id = (int) par;
24 
25     /*go to sleep*/
26 for(int i = 0; i < 1000; ++i){
27     while(1 == __sync_val_compare_and_swap(&futex_addr, 0, 1) ){
28         futex_wait(&futex_addr,1);
29     }
30     ++num;
31     futex_addr = 0;
32     futex_wake(&futex_addr, NUM);
33 }
34   //      printf("Thread %d starting to work!\n",id);
35         return NULL;
36 }
37 
38 int main(){
39         pthread_t threads[NUM];
40         int i;
41 
42         printf("Everyone go...\n");
43         float time_use=0;
44         struct timeval start;
45         struct timeval end;
46         gettimeofday(&start,NULL);
47 
48 
49 
50         for (i=0;i<NUM;i++){
51                 pthread_create(&threads[i],NULL,thread_f,(void *)i);
52         }
53 
54     /*wake threads*/
55 
56     /*give the threads time to complete their tasks*/
57         for (i=0;i<NUM;i++){
58                 pthread_join(*(threads + i), NULL);
59         }
60 
61 
62     printf("Main is quitting...\n");
63     printf("and num is %d\n", num);
64 
65     gettimeofday(&end,NULL);
66     time_use=(end.tv_sec-start.tv_sec)+(end.tv_usec-start.tv_usec) / 1000000.0;//微秒
67     printf("time_use is %f \n",time_use);
68     return 0;
69 }

执行输出为:

Everyone go...
Main is quitting...
and num is 1000000
time_use is 0.283753

1000个线程执行1000次+1,答案为1000000正确。

我们尝试将futex_wait中sys_call做一下修改:

int futex_wait(void* addr, int val){
//  return syscall(SYS_futex, addr, FUTEX_WAIT, val, NULL, NULL, 0);
    return INTERNAL_SYSCALL_NCS(addr, FUTEX_WAIT, val, NULL);
}

然后添加宏INTERNAL_SYSCALL_NCS:

#define INTERNAL_SYSCALL_NCS(a1, a2, a3, a4)  \
  ({                                              unsigned long long int resultvar;              long int __arg4 = (long int) (a4);                             long int __arg3 = (long int) (a3);                             long int __arg2 = (long int) (a2);                             long int __arg1 = (long int) (a1);                         \    
    register long int _a4 asm ("r10") = __arg4;                        register long int _a3 asm ("rdx") = __arg3;                        register long int _a2 asm ("rsi") = __arg2;                        register long int _a1 asm ("rdi") = __arg1;                        asm volatile (     "syscall\n\t"                                     : "=a" (resultvar)                                    : "0" (SYS_futex), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4) : "memory", "cc", "r11", "cx");       (long long int) resultvar; })

得到如下代码:

 1 #include <stdio.h>
 2 #include <pthread.h>
 3 #include <linux/futex.h>
 4 #include <syscall.h>
 5 #include <unistd.h>
 6 #include <sys/time.h>
 7 
 8 
 9 #define NUM 1000
10 
11 #define INTERNAL_SYSCALL_NCS(a1, a2, a3, a4)  12   ({                                          13     unsigned long long int resultvar;          14     long int __arg4 = (long int) (a4);                         15     long int __arg3 = (long int) (a3);                         16     long int __arg2 = (long int) (a2);                         17     long int __arg1 = (long int) (a1);                         \    
18     register long int _a4 asm ("r10") = __arg4;                    19     register long int _a3 asm ("rdx") = __arg3;                    20     register long int _a2 asm ("rsi") = __arg2;                    21     register long int _a1 asm ("rdi") = __arg1;                    22     asm volatile ( 23     "syscall\n\t"                                 24     : "=a" (resultvar)                                25     : "0" (SYS_futex), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4) : "memory", "cc", "r11", "cx");   26     (long long int) resultvar; })
27 
28 
29 int num = 0;
30 int futex_addr = 0;
31 
32 int futex_wait(void* addr, int val){
33 //  return syscall(SYS_futex, addr, FUTEX_WAIT, val, NULL, NULL, 0);
34     return INTERNAL_SYSCALL_NCS(addr, FUTEX_WAIT, val, NULL);
35 }
36 int futex_wake(void* addr, int val){
37   return syscall(SYS_futex, addr, FUTEX_WAKE, val, NULL, NULL, 0);
38 }
39 
40 void* thread_f(void* par){
41         int id = (int) par;
42 
43     /*go to sleep*/
44 for(int i = 0; i < 1000; ++i){
45     while(1 == __sync_val_compare_and_swap(&futex_addr, 0, 1) ){
46         futex_wait(&futex_addr,1);
47     }
48     ++num;
49     futex_addr = 0;
50     futex_wake(&futex_addr, NUM);
51 }
52   //      printf("Thread %d starting to work!\n",id);
53         return NULL;
54 }
55 
56 int main(){
57         pthread_t threads[NUM];
58         int i;
59 
60         printf("Everyone go...\n");
61         float time_use=0;
62         struct timeval start;
63         struct timeval end;
64         gettimeofday(&start,NULL);
65 
66 
67 
68         for (i=0;i<NUM;i++){
69                 pthread_create(&threads[i],NULL,thread_f,(void *)i);
70         }
71 
72     /*wake threads*/
73 
74     /*give the threads time to complete their tasks*/
75         for (i=0;i<NUM;i++){
76                 pthread_join(*(threads + i), NULL);
77         }
78 
79 
80     printf("Main is quitting...\n");
81     printf("and num is %d\n", num);
82 
83     gettimeofday(&end,NULL);
84     time_use=(end.tv_sec-start.tv_sec)+(end.tv_usec-start.tv_usec) / 1000000.0;//微秒
85     printf("time_use is %f \n",time_use);
86     return 0;
87 }

注意到我们这里与pthread不一样的地方在于

1 == __sync_val_compare_and_swap(&futex_addr, 0, 1)

注意到我们这里的和pthread_mutex不一样的地方在于我们是原子得将值futex_addr从0改为1.

执行如上代码,输出为:

Everyone go...
Main is quitting...
and num is 1000000
time_use is 0.254833

答案同样是1000000,所以这个采用汇编形式的调用符合了我们的预期,应该是和系统调用一致的。

我们之后接着来看看pthread_mutex_unlock的实现。


 

pthread_mutex_lock/unlock(1)