Day 02 链锁

《POSIX多线程程序设计》

随意顺序解锁互斥量，不会导致死锁的发生。但在使用“试锁-回退”算法时，应该总以相反的顺序，解锁互斥量。这样有利于，减少线程做回退操作的可能性。

 

链锁及其使用场景

 

申请多锁的代码示例： 使用backoff 和 yield_flag 命令行参数来调整，增加发生冲突的可能性。

/*************************************************************************
 *  File Name: backoff.c
 *  Created Time: Thu 30 Nov 2017 05:56:18 PM CST
 *************************************************************************/

#include <stdio.h>
#include <pthread.h>
#include <sched.h>
#include "../errors/errors.h"

#define ITERATIONS 10

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

int backoff = 1;         /* Whether to backofff or deadlock*/
int yield_flag = 0;        /* 0: no yield, >0: yield, <0: sleep */

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

    for(iterator=0; iterator<ITERATIONS; ++iterator) {
        backoffs=0;
        for(i=0;i<3;i++) {
            if(i==0){
                status = pthread_mutex_lock(&mutex[i]);
                if(status !=0)
                    err_abort(status,"First lock");
            } else {
                if(backoff) {
                    status = pthread_mutex_trylock(&mutex[i]);
                } else
                    status = pthread_mutex_lock(&mutex[i]);

                if(status == EBUSY) {
                    backoffs++;
                    DPRINTF((
                                " [forward locker backing off at %d]\n",i));
                    for(;i>=0;i--) {
                        status = pthread_mutex_unlock(&mutex[i]);
                        if(status != 0)
                            err_abort(status,"backoff");
                    }

                } else {
                    if(status != 0)
                        err_abort(status,"Lock mutex");
                    DPRINTF (("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);
        for(i=2;i>=0;i--) {
            pthread_mutex_unlock(&mutex[i]);
        }
        sched_yield();
    }
    return NULL;
}

void *lock_backward(void *arg)
{

    int i, iterator, backoffs;
    int status;

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

                if(status == EBUSY) {
                    backoffs++;
                    DPRINTF((
                                " [backward locker backing off at %d]\n",i));
                    for(;i<3;i++) {
                        status = pthread_mutex_unlock(&mutex[i]);
                        if(status != 0)
                            err_abort(status,"backoff");
                    }

                } else {
                    if(status != 0)
                        err_abort(status,"Lock mutex");
                    DPRINTF (("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);
        for(i=0;i<=2;i++) {
            pthread_mutex_unlock(&mutex[i]);
        }
        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)
        err_abort(status,"create forward");
    status = pthread_create(
            &backward,NULL,lock_backward,NULL);
    if(status != 0)
        err_abort(status,"create backward");

    pthread_exit(NULL);
}