深入Phtread(二)：线程的同步-Mutex

并行的世界，没有同步，就失去了秩序，就会乱作一团！试想，交通没有红绿灯，生产线产品装配没有一定的顺序... 结果是显而易见的。多个线程也需要同步，否则程序运行起来结果不可预测，这是我们最不能容忍的。交通的同步机制就是红绿灯，Pthread提供了互斥量（mutex）和条件变量（Condition Variables）两种机制去同步线程。
不变量，临界区和判定条件
互斥量（Mutex）
创建和销毁互斥量
锁定和解锁
调整mutex大小
使用多个mutex
锁定链
不变量，临界区和判定条件
不变量（Invariant）：程序所做的一些假设，特别是指变量之间的关系。如：一个queue，有头节点，和其它数据节点，这些元素之间的连接关系就是不变量。当程序里面不变量遭受破坏时，后果往往是很严重的，轻则数据出错，重则程序直接崩溃。

临界区（Critical Section）：处理共享数据的一段代码。

判定条件（Predicates）：描述不变量状态的逻辑表达式。

互斥量（Mutex）
一般，多个线程之间都会共享一些数据，当多个线程同时访问操作这些共享数据时。问题出来了，一个线程正在修改数据时，另外一个可能也去操作这些数据，结果就会变得不一致了。如(gv=0是共享的数据)：

线程A：a = gv; gv = a + 10;
线程B: b = gv; gv = a + 100;

可能发生A执行完a=gv(0)时，B开始执行b=gv(0); gv=a+100，此时gv=100，然后a执行gv=a+10，最后gv=10。并不是我们要的结果，我们的想法是两个线程并发的给gv加上一个值，期望结果110。^_^ 若这是你银行卡的余额，若没有同步，那就惨了（你往卡里打钱，你有个朋友也同时往你卡里汇钱，很有可能余额只仅加上一方打的）。

互斥量就是为了解决这种问题而设计的，它是Dijkstra信号量的一种特殊形式。它使得线程可以互斥地访问共享数据。如：


上图展示了三个线程共享一个互斥量，位于矩形中心线下方的线程锁定了该互斥量；位于中心线上方且在矩形范围内的线程等待该互斥量被解锁，出于阻塞状态，在矩形外面的线程正常运行。刚开始，mutex是解锁的，线程1成功将其锁定，据为己有，因为并没有其它线程拥有它。然后，线程2尝试去锁定，发现被线程1占用，所以阻塞于此，等到线程1解锁了该mutex，线程2立马将mutex锁定。过了会，线程3尝试去锁定mutex，由于mutex被锁定，所以阻塞于此。线程1调用pthread_mutex_trylock尝试去锁定个mutex，发现该mutex被锁定，自己返回继续执行，并没有阻塞。继续线程2解锁，线程3锁定成功，最后线程3完成任务解锁mutex。

创建和销毁互斥量
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int pthread_mutex_init(pthread_mutex_t* mutex, pthread_mutexattr_t* attr);
int pthread_mutex_destroy(pthread_mutex_t* mutex);

不要尝试去使用复制的的mutex，结果未定义。

静态创建，当mutex以extern或者static存储时，可以用PTHREAD_MUTEX_INITIALIZER初始化，此时该mutex使用默认属性。
+ expand sourceview plaincopy to clipboardprint?
#include "error.h"
#include <pthread.h>

typedef struct my_struct_tag
{
pthread_mutex_t mutex;
int value;
} my_struct_t;

my_struct_t data = { PTHREAD_MUTEX_INITIALIZER, 0};

int main()
{
return 0;
}
#include "error.h"
#include <pthread.h>

typedef struct my_struct_tag
{
pthread_mutex_t mutex;
int value;
} my_struct_t;

my_struct_t data = { PTHREAD_MUTEX_INITIALIZER, 0};

int main()
{
return 0;
}

动态创建，往往使用mutex时，都会将它和共享数据绑在一起，此时就需要pthread_mutex_init去动态初始化了，记得用完后pthread_mutex_destroy。
+ expand sourceview plaincopy to clipboardprint?
#include "error.h"
#include <pthread.h>

typedef struct my_struct_tag
{
pthread_mutex_t mutex;
int value;
} my_struct_t;

int main()
{
my_struct_t* data;
int status;

data = (my_struct_t*)malloc(sizeof(my_struct_t));
status = pthread_mutex_init(&data->mutex, NULL);
if(status != 0)
ERROR_ABORT(status, "pthread_mutex_init");

pthread_mutex_destroy(&data->mutex);
free(data);

return 0;
}
#include "error.h"
#include <pthread.h>

typedef struct my_struct_tag
{
pthread_mutex_t mutex;
int value;
} my_struct_t;

int main()
{
my_struct_t* data;
int status;

data = (my_struct_t*)malloc(sizeof(my_struct_t));
status = pthread_mutex_init(&data->mutex, NULL);
if(status != 0)
ERROR_ABORT(status, "pthread_mutex_init");

pthread_mutex_destroy(&data->mutex);
free(data);

return 0;
}

锁定和解锁
原则见上面。
int pthread_mutex_lock(pthread_mutex_t* mutex);
int pthread_mutex_trylock(pthread_mutex_t* mutex);
int pthread_mutex_unlock(pthread_mutex_t* mutex);
+ expand sourceview plaincopy to clipboardprint?
#include <pthread.h>
#include <sys/types.h>
#include "error.h"
#include <errno.h>

#define SPIN 10000000

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
long counter;
time_t end_time;

void* counter_thread(void* arg)
{
int status;
int spin;

while(time(NULL) < end_time)
{
status = pthread_mutex_lock(&mutex);
if(status != 0)
ERROR_ABORT(status, "Lock mutex");

for(spin = 0; spin < SPIN; spin++)
counter++;

status = pthread_mutex_unlock(&mutex);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
sleep(1);
}

printf("Coutner is %#lx/n", counter);

return NULL;
}

void* monitor_thread(void* arg)
{
int status;
int misses = 0;

while(time(NULL) < end_time)
{
sleep(3);

status = pthread_mutex_trylock(&mutex);
if(status != EBUSY)
{
if(status != 0)
ERROR_ABORT(status, "Trylock mutex");

printf("Counter is %ld/n", counter/SPIN);
status = pthread_mutex_unlock(&mutex);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
}else
misses++;
}
printf("Monitro thread missed update %d times./n", misses);
return NULL;
}

int main()
{
int status;
pthread_t pid_counter;
pthread_t pid_monitor;

end_time = time(NULL) + 60;

status = pthread_create(&pid_counter, NULL, counter_thread, NULL);
if(status != 0)
ERROR_ABORT(status, "fail to create thread counter");

status = pthread_create(&pid_monitor, NULL, monitor_thread, NULL);
if(status != 0)
ERROR_ABORT(status, "fail to create monitor thread");

status = pthread_join(pid_counter, NULL);
if(status != 0 )
ERROR_ABORT(status, "fail to join counter thread");

status = pthread_join(pid_monitor, NULL);
if(status != 0)
ERROR_ABORT(status, "fail to join monitor thread");

return 0;
}
#include <pthread.h>
#include <sys/types.h>
#include "error.h"
#include <errno.h>

#define SPIN 10000000

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
long counter;
time_t end_time;

void* counter_thread(void* arg)
{
int status;
int spin;

while(time(NULL) < end_time)
{
status = pthread_mutex_lock(&mutex);
if(status != 0)
ERROR_ABORT(status, "Lock mutex");

for(spin = 0; spin < SPIN; spin++)
counter++;

status = pthread_mutex_unlock(&mutex);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
sleep(1);
}

printf("Coutner is %#lx/n", counter);

return NULL;
}

void* monitor_thread(void* arg)
{
int status;
int misses = 0;

while(time(NULL) < end_time)
{
sleep(3);

status = pthread_mutex_trylock(&mutex);
if(status != EBUSY)
{
if(status != 0)
ERROR_ABORT(status, "Trylock mutex");

printf("Counter is %ld/n", counter/SPIN);
status = pthread_mutex_unlock(&mutex);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
}else
misses++;
}
printf("Monitro thread missed update %d times./n", misses);
return NULL;
}

int main()
{
int status;
pthread_t pid_counter;
pthread_t pid_monitor;

end_time = time(NULL) + 60;

status = pthread_create(&pid_counter, NULL, counter_thread, NULL);
if(status != 0)
ERROR_ABORT(status, "fail to create thread counter");

status = pthread_create(&pid_monitor, NULL, monitor_thread, NULL);
if(status != 0)
ERROR_ABORT(status, "fail to create monitor thread");

status = pthread_join(pid_counter, NULL);
if(status != 0 )
ERROR_ABORT(status, "fail to join counter thread");

status = pthread_join(pid_monitor, NULL);
if(status != 0)
ERROR_ABORT(status, "fail to join monitor thread");

return 0;
}

调整mutex大小
mutex应该多大？这里的大小是相对的，如mutex锁定到解锁之间的代码只有一行，比起有10行的就小了。 原则是：尽可能大，但不要太大（As big as neccessary, but no bigger）。考虑下面的因素：1> mutex并不是免费的，是有开销的，不要太小了，太小了程序只忙于锁定和解锁了。2> mutex锁定的区域是线性执行的，若太大了，没有发挥出并发的优越性。
3> 自己掂量1和2，根据实际情况定，或者尝试着去做。
使用多个mutex
使用多个mutex一定要注意，防止死锁（deadlock）发生。下面是一个典型死锁：

线程A：pthread_mutex_lock(&mutex_a); pthread_mutex_lock(&mutex_b); ...
线程B：pthread_mutex_lock(&mutex_b); pthread_mutex_lock(&mutex_a); ...

存在这种可能，线程A执行了第一句，锁定了mutex_a；然后线程开始执行第一句锁定mutex_b；然后他们互相等待解锁mutex，A等mutex_b被解锁，B等mutex_a被解锁，不肯让步，出于死锁状态。
+ expand sourceview plaincopy to clipboardprint?
#include <pthread.h>
#include "error.h"
#include <time.h>

pthread_mutex_t mutex_a = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t mutex_b = PTHREAD_MUTEX_INITIALIZER;

void* thread1(void* arg)
{
while(1)
{
/*sleep(1);*/
pthread_mutex_lock(&mutex_a);
pthread_mutex_lock(&mutex_b);

printf("[%lu]thread 1 is running! /n", time(NULL));

pthread_mutex_unlock(&mutex_b);
pthread_mutex_unlock(&mutex_a);
}
return NULL;
}

void* thread2(void* arg)
{
while(1)
{
/*sleep(1);*/

pthread_mutex_lock(&mutex_b);
pthread_mutex_lock(&mutex_a);

printf("[%lu]thread 2 is running! /n",time(NULL));

pthread_mutex_unlock(&mutex_a);
pthread_mutex_unlock(&mutex_b);

}
return NULL;
}

int main()
{
pthread_t tid1, tid2;
int status;

status = pthread_create(&tid1, NULL, thread1, NULL);
if(status != 0)
ERROR_ABORT(status, "thread 1");

status = pthread_create(&tid2, NULL, thread2, NULL);
if(status !=0)
ERROR_ABORT(status, "thread 2");

status = pthread_join(tid1, NULL);
if(status != 0)
ERROR_ABORT(status, "join thread1");

status = pthread_join(tid2, NULL);
if(status != 0)
ERROR_ABORT(status, "join thread2");
}
#include <pthread.h>
#include "error.h"
#include <time.h>

pthread_mutex_t mutex_a = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t mutex_b = PTHREAD_MUTEX_INITIALIZER;

void* thread1(void* arg)
{
while(1)
{
/*sleep(1);*/
pthread_mutex_lock(&mutex_a);
pthread_mutex_lock(&mutex_b);

printf("[%lu]thread 1 is running! /n", time(NULL));

pthread_mutex_unlock(&mutex_b);
pthread_mutex_unlock(&mutex_a);
}
return NULL;
}

void* thread2(void* arg)
{
while(1)
{
/*sleep(1);*/

pthread_mutex_lock(&mutex_b);
pthread_mutex_lock(&mutex_a);

printf("[%lu]thread 2 is running! /n",time(NULL));

pthread_mutex_unlock(&mutex_a);
pthread_mutex_unlock(&mutex_b);

}
return NULL;
}

int main()
{
pthread_t tid1, tid2;
int status;

status = pthread_create(&tid1, NULL, thread1, NULL);
if(status != 0)
ERROR_ABORT(status, "thread 1");

status = pthread_create(&tid2, NULL, thread2, NULL);
if(status !=0)
ERROR_ABORT(status, "thread 2");

status = pthread_join(tid1, NULL);
if(status != 0)
ERROR_ABORT(status, "join thread1");

status = pthread_join(tid2, NULL);
if(status != 0)
ERROR_ABORT(status, "join thread2");
}

解决死锁的方法：
固定锁定顺序（Fixed locking hierarchy）：锁定mutex的顺序固定。
线程A：pthread_mutex_lock(&mutex_a); pthread_mutex_lock(&mutex_b); ...
线程B：pthread_mutex_lock(&mutex_a); pthread_mutex_lock(&mutex_b); ...

尝试和回退（Try and back off）： 锁定第一个后，尝试锁定下一个，若锁定成功，继续尝试下一个，若锁定失败，解锁先去锁定的。

解锁顺序不会引起死锁.
+ expand sourceview plaincopy to clipboardprint?
#include <pthread.h>
#include "error.h"
#include <errno.h>

#define ITERATIONS 100

pthread_mutex_t mutex[3] = {
PTHREAD_MUTEX_INITIALIZER,
PTHREAD_MUTEX_INITIALIZER,
PTHREAD_MUTEX_INITIALIZER
};

int backoff = 1;
int yield_flag = 0;

void* lock_forward(void* arg)
{
int i, iterate, backoffs;
int status;

for(iterate = 0; iterate < ITERATIONS; iterate++)
{
backoffs = 0;
for(i = 0; i < 3; i++){
if(i == 0)
{
status = pthread_mutex_lock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status,"Lock mutex");
}else
{
if(backoff)
status = pthread_mutex_trylock(&mutex[i]);
else
status = pthread_mutex_lock(&mutex[i]);

if(status == EBUSY)
{
backoff++;
printf("forward locker backing off at %d./n", i);
for(; i >= 0; i--)
{
status = pthread_mutex_unlock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
}
}else
{
if(status != 0)
ERROR_ABORT(status, "Lock mutex");

printf("forward locker got %d /n", i);
}
}

if(yield_flag){
if(yield_flag > 0)
sched_yield();
else
sleep(1);
}
}

printf("lock forward got all locks , %d backoffs/n", backoffs);

pthread_mutex_unlock(&mutex[2]);
pthread_mutex_unlock(&mutex[1]);
pthread_mutex_unlock(&mutex[0]);
sched_yield();
}

return NULL;
}

void* lock_backward(void* arg)
{
int i, iterate, backoffs;
int status;

for(iterate = 0; iterate < ITERATIONS; iterate++)
{
backoffs = 0;
for(i = 2; i >= 0; i--){
if(i == 2)
{
status = pthread_mutex_lock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status,"Lock mutex");
}else
{
if(backoff)
status = pthread_mutex_trylock(&mutex[i]);
else
status = pthread_mutex_lock(&mutex[i]);

if(status == EBUSY)
{
backoff++;
printf("backward locker backing off at %d./n", i);
for(; i < 3; i++)
{
status = pthread_mutex_unlock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
}
}else
{
if(status != 0)
ERROR_ABORT(status, "Lock mutex");

printf("backward locker got %d /n", i);
}
}

if(yield_flag){
if(yield_flag > 0)
sched_yield();
else
sleep(1);
}
}

printf("lock backward got all locks , %d backoffs/n", backoffs);

pthread_mutex_unlock(&mutex[0]);
pthread_mutex_unlock(&mutex[1]);
pthread_mutex_unlock(&mutex[2]);
sched_yield();
}

return NULL;
}

int main(int argc, char* argv[])
{
pthread_t forward, backward;
int status;

if(argc > 1)
backoff = atoi(argv[1]);

if(argc > 2)
yield_flag = atoi(argv[2]);

status = pthread_create(&forward, NULL, lock_forward, NULL);
if(status != 0)
ERROR_ABORT(status, "Create forward");

status = pthread_create(&backward, NULL, lock_backward, NULL);
if(status != 0)
ERROR_ABORT(status, "Create backward");

pthread_exit(NULL);
}
#include <pthread.h>
#include "error.h"
#include <errno.h>

#define ITERATIONS 100

pthread_mutex_t mutex[3] = {
PTHREAD_MUTEX_INITIALIZER,
PTHREAD_MUTEX_INITIALIZER,
PTHREAD_MUTEX_INITIALIZER
};

int backoff = 1;
int yield_flag = 0;

void* lock_forward(void* arg)
{
int i, iterate, backoffs;
int status;

for(iterate = 0; iterate < ITERATIONS; iterate++)
{
backoffs = 0;
for(i = 0; i < 3; i++){
if(i == 0)
{
status = pthread_mutex_lock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status,"Lock mutex");
}else
{
if(backoff)
status = pthread_mutex_trylock(&mutex[i]);
else
status = pthread_mutex_lock(&mutex[i]);

if(status == EBUSY)
{
backoff++;
printf("forward locker backing off at %d./n", i);
for(; i >= 0; i--)
{
status = pthread_mutex_unlock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
}
}else
{
if(status != 0)
ERROR_ABORT(status, "Lock mutex");

printf("forward locker got %d /n", i);
}
}

if(yield_flag){
if(yield_flag > 0)
sched_yield();
else
sleep(1);
}
}

printf("lock forward got all locks , %d backoffs/n", backoffs);

pthread_mutex_unlock(&mutex[2]);
pthread_mutex_unlock(&mutex[1]);
pthread_mutex_unlock(&mutex[0]);
sched_yield();
}

return NULL;
}

void* lock_backward(void* arg)
{
int i, iterate, backoffs;
int status;

for(iterate = 0; iterate < ITERATIONS; iterate++)
{
backoffs = 0;
for(i = 2; i >= 0; i--){
if(i == 2)
{
status = pthread_mutex_lock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status,"Lock mutex");
}else
{
if(backoff)
status = pthread_mutex_trylock(&mutex[i]);
else
status = pthread_mutex_lock(&mutex[i]);

if(status == EBUSY)
{
backoff++;
printf("backward locker backing off at %d./n", i);
for(; i < 3; i++)
{
status = pthread_mutex_unlock(&mutex[i]);
if(status != 0)
ERROR_ABORT(status, "Unlock mutex");
}
}else
{
if(status != 0)
ERROR_ABORT(status, "Lock mutex");

printf("backward locker got %d /n", i);
}
}

if(yield_flag){
if(yield_flag > 0)
sched_yield();
else
sleep(1);
}
}

printf("lock backward got all locks , %d backoffs/n", backoffs);

pthread_mutex_unlock(&mutex[0]);
pthread_mutex_unlock(&mutex[1]);
pthread_mutex_unlock(&mutex[2]);
sched_yield();
}

return NULL;
}

int main(int argc, char* argv[])
{
pthread_t forward, backward;
int status;

if(argc > 1)
backoff = atoi(argv[1]);

if(argc > 2)
yield_flag = atoi(argv[2]);

status = pthread_create(&forward, NULL, lock_forward, NULL);
if(status != 0)
ERROR_ABORT(status, "Create forward");

status = pthread_create(&backward, NULL, lock_backward, NULL);
if(status != 0)
ERROR_ABORT(status, "Create backward");

pthread_exit(NULL);
}

锁定链

一般用于遍历数据结果（树，链表），一个用于锁定指针，一个锁定数据。
形如：
pthread_mutex_lock(&mutex_a);
pthread_mutex_lock(&mutex_b);
...
pthread_mutex_unlock(&mutex_a)
...
pthread_mutex_unlock(&mutex_b)

注意，锁定链往往会出现大量的锁定和解锁操作，有时会得不偿失。

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/future_fighter/archive/2009/02/11/3878386.aspx