求教APUE中的一个问题

zcspdf

11.2 In the example shown in Figure 11.13, what additional synchronization (if any) is necessary to allow the master thread to change the thread ID associated with a pending job? How would this affect the job_remove function?


这个问题的意思似乎是增加一个函数，来改变队列里面job的j_id，那只要先加写锁，find job，改id，然后在释放锁就行
为什么还要影响到job_remove这个函数呢？


Figure 11.13. Using readerwriter locks
#include <stdlib.h>
#include <pthread.h>

struct job {
    struct job *j_next;
    struct job *j_prev;
    pthread_t   j_id;   /* tells which thread handles this job */
    /* ... more stuff here ... */
};

struct queue {
    struct job      *q_head;
    struct job      *q_tail;
    pthread_rwlock_t q_lock;
};

/*
* Initialize a queue.
*/
int
queue_init(struct queue *qp)
{
    int err;

    qp->q_head = NULL;
    qp->q_tail = NULL;
    err = pthread_rwlock_init(&qp->q_lock, NULL);
    if (err != 0)
        return(err);

    /* ... continue initialization ... */

    return(0);
}

/*
* Insert a job at the head of the queue.
*/
void
job_insert(struct queue *qp, struct job *jp)
{
    pthread_rwlock_wrlock(&qp->q_lock);
    jp->j_next = qp->q_head;
    jp->j_prev = NULL;
    if (qp->q_head != NULL)
        qp->q_head->j_prev = jp;
    else
        qp->q_tail = jp;      /* list was empty */
    qp->q_head = jp;
    pthread_rwlock_unlock(&qp->q_lock);
}

/*
* Append a job on the tail of the queue.
*/
void
job_append(struct queue *qp, struct job *jp)
{
    pthread_rwlock_wrlock(&qp->q_lock);
    jp->j_next = NULL;
    jp->j_prev = qp->q_tail;
    if (qp->q_tail != NULL)
        qp->q_tail->j_next = jp;
    else
        qp->q_head = jp;   /* list was empty */
    qp->q_tail = jp;
    pthread_rwlock_unlock(&qp->q_lock);
}

/*
* Remove the given job from a queue.
*/
void
job_remove(struct queue *qp, struct job *jp)
{
    pthread_rwlock_wrlock(&qp->q_lock);
    if (jp == qp->q_head) {
        qp->q_head = jp->j_next;
        if (qp->q_tail == jp)
            qp->q_tail = NULL;
    } else if (jp == qp->q_tail) {
        qp->q_tail = jp->j_prev;
        if (qp->q_head == jp)
            qp->q_head = NULL;
    } else {
        jp->j_prev->j_next = jp->j_next;
        jp->j_next->j_prev = jp->j_prev;
    }
    pthread_rwlock_unlock(&qp->q_lock);
}
/*
* Find a job for the given thread ID.
*/
struct job *
job_find(struct queue *qp, pthread_t id)
{
    struct job *jp;

    if (pthread_rwlock_rdlock(&qp->q_lock) != 0)
        return(NULL);

    for (jp = qp->q_head; jp != NULL; jp = jp->j_next)
         if (pthread_equal(jp->j_id, id))
             break;

    pthread_rwlock_unlock(&qp->q_lock);
    return(jp);
}

lenovo

书后面答案，没看到？