[英]Safe removal of a node in a concurrent linked list
我正在读这本书APUE。 当我阅读关于pthread reader / writer-lock的章节时,我对使用reader / writer-lock实现并发队列有一个疑问。
struct queue {
struct job *q_head;
struct job *q_tail;
pthread_rwlock_t 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
jp->j_next->j_prev = jp->j_prev;
} else if (jp == qp->q_tail) {
qp->q_tail = jp->j_prev;
jp->j_prev->j_next = jp->j_next;
} else {
jp->j_prev->j_next = jp->j_next;
jp->j_next->j_prev = jp->j_prev;
}
pthread_rwlock_unlock(&qp->q_lock);
}
我的问题是这个实现如何确保只从链表中删除一次struct job
。 根据我的理解,可以安排两个线程,使它们就在pthread_rwlock_wrlock
行之前。 然后struct job *jp
可能会被释放两次。 如果struct job *
是动态分配的数据结构,这可能会导致双重错误。 有什么建议?
您的代码中存在竞争条件。 其他更改可能发生在两个线程之间的队列中,该队列获取队列上的写锁定,然后尝试删除同一节点。 可以删除其他节点,可以添加其他节点。 因此,如果线程A删除节点,则发生更改,然后线程B再次尝试删除同一节点,您的队列可能已损坏。
代码需要信息才能让它知道节点已被删除。
查看添加的注释以及修复竞争条件的代码:
struct queue {
struct job *q_head;
struct job *q_tail;
pthread_rwlock_t q_lock;
};
/*
* Remove the given job from a queue.
*/
void
job_remove(struct queue *qp, struct job *jp)
{
// we assume here that jp is actually in the queue
pthread_rwlock_wrlock(&qp->q_lock);
// at this point, jp may no longer be in the queue,
// and in fact, the queue may be completely different.
// thus, any modification to the queue based on the
// assumption that jp is still part of the queue
// can lead to corruption of the queue
// so check if jp has been removed - we'll later set
// both j_next and j_prev to NULL after jp is
// removed from the queue - and if they're both
// NULL here that means another thread already
// removed jp from the queue
if ( !(jp->j_next) && !(jp->j_prev) ) {
// empty statement - jp is already removed
;
}
else if (jp == qp->q_head) {
qp->q_head = jp->j_next;
if (qp->q_tail == jp)
qp->q_tail = NULL;
else
jp->j_next->j_prev = jp->j_prev;
} // and this brace was missing in your posted code...
else if (jp == qp->q_tail) {
qp->q_tail = jp->j_prev;
jp->j_prev->j_next = jp->j_next;
} else {
jp->j_prev->j_next = jp->j_next;
jp->j_next->j_prev = jp->j_prev;
}
// make sure data in jp no longer refers to
// the queue - this will also tell any other
// thread that accesses jp that it's already
// been removed
jp->j_next = NULL;
jp->j_prev = NULL;
pthread_rwlock_unlock(&qp->q_lock);
}
您还需要检查jp如何获得free()
d或delete
d。 不允许多个线程这样做。
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