BIND9采用的是事件驱动的机制来工作，而事件的源头则是IO，IO在linux使用的EPOLL的边缘触发模式。
本篇说的是epoll，BIND9如果创建了watcher线程（宏USE_WATCHER_THREAD控制），这里就讨论有线程的情况，实际上即使不创建
线程干的也都是一样的活。在lib/isc/socket.c中setup_watcher函数：（所有的代码都是截取的epoll下的片段，因为还有kqueue，devpoll，select等的实现代码，太多了）
#elif defined(USE_EPOLL)
manager->nevents = ISC_SOCKET_MAXEVENTS;
manager->events = isc_mem_get(mctx, sizeof(struct epoll_event) *
manager->nevents);
if (manager->events == NULL)
return (ISC_R_NOMEMORY);
manager->epoll_fd = epoll_create(manager->nevents);
if (manager->epoll_fd == -1) {
result = isc__errno2result(errno);
isc__strerror(errno, strbuf, sizeof(strbuf));
UNEXPECTED_ERROR(__FILE__, __LINE__,
"epoll_create %s: %s",
isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
ISC_MSG_FAILED, "failed"),
strbuf);
isc_mem_put(mctx, manager->events,
sizeof(struct epoll_event) * manager->nevents);
return (result);
}
#ifdef USE_WATCHER_THREAD
result = watch_fd(manager, manager->pipe_fds[0], SELECT_POKE_READ);
if (result != ISC_R_SUCCESS) {
close(manager->epoll_fd);
isc_mem_put(mctx, manager->events,
sizeof(struct epoll_event) * manager->nevents);
return (result);
}
#endif /* USE_WATCHER_THREAD */
先是创建了要监视的最大socket fd数目（manager->nevents）对应的epoll_event结构体数组，然后调用epoll_create函数创建一个epoll fd，参数则是指定监视的socket fd
最大数目。我的内核版本是3.13，man一下epoll_create发现它是这样说的：epoll_create() creates an epoll(7) instance. Since Linux 2.6.8, thesize argument is ignored, but must be greater than zero。这个函数在2.6.8内核以后就忽略参数size了，但是传递的参数值一定要大于0。后来找了一下资料，网上的高手的博客说的就很清楚了http://www.cnblogs.com/apprentice89/p/3234677.html。继续往下说，后面的watch_fd实在创建线程的情况下才有，就是将pipe_fds[0]这个管道描述符，也就是一个可读的流，而上述的socket fd都是可以归为流。watch_fd的实现代码：
#elif defined(USE_EPOLL)
struct epoll_event event;
if (msg == SELECT_POKE_READ)
event.events = EPOLLIN;
else
event.events = EPOLLOUT;
memset(&event.data, 0, sizeof(event.data));
event.data.fd = fd;
if (epoll_ctl(manager->epoll_fd, EPOLL_CTL_ADD, fd, &event) == -1 &&
errno != EEXIST) {
result = isc__errno2result(errno);
}
return (result);
这是将pipe_fds[0]加入epoll_fd的监听队列，EPOLL_CTL_ADD是操作类型，注册该fd到epoll_fd上。这个管道的目的是接收管理该线程的消息，比如线程退出。
那么进入线程看：
static isc_threadresult_t
watcher(void *uap) {
isc__socketmgr_t *manager = uap;
isc_boolean_t done;
int ctlfd;
int cc;
#ifdef USE_KQUEUE
const char *fnname = "kevent()";
#elif defined (USE_EPOLL)
const char *fnname = "epoll_wait()";
#elif defined(USE_DEVPOLL)
const char *fnname = "ioctl(DP_POLL)";
struct dvpoll dvp;
#elif defined (USE_SELECT)
const char *fnname = "select()";
int maxfd;
#endif
char strbuf[ISC_STRERRORSIZE];
#ifdef ISC_SOCKET_USE_POLLWATCH
pollstate_t pollstate = poll_idle;
#endif
/*
* Get the control fd here. This will never change.
*/
ctlfd = manager->pipe_fds[0];
done = ISC_FALSE;
while (!done) {
do {
#ifdef USE_KQUEUE
cc = kevent(manager->kqueue_fd, NULL, 0,
manager->events, manager->nevents, NULL);
#elif defined(USE_EPOLL)
cc = epoll_wait(manager->epoll_fd, manager->events,
manager->nevents, -1);
#elif defined(USE_DEVPOLL)
dvp.dp_fds = manager->events;
dvp.dp_nfds = manager->nevents;
#ifndef ISC_SOCKET_USE_POLLWATCH
dvp.dp_timeout = -1;
#else
if (pollstate == poll_idle)
dvp.dp_timeout = -1;
else
dvp.dp_timeout = ISC_SOCKET_POLLWATCH_TIMEOUT;
#endif /* ISC_SOCKET_USE_POLLWATCH */
cc = ioctl(manager->devpoll_fd, DP_POLL, &dvp);
#elif defined(USE_SELECT)
LOCK(&manager->lock);
memcpy(manager->read_fds_copy, manager->read_fds,
manager->fd_bufsize);
memcpy(manager->write_fds_copy, manager->write_fds,
manager->fd_bufsize);
maxfd = manager->maxfd + 1;
UNLOCK(&manager->lock);
cc = select(maxfd, manager->read_fds_copy,
manager->write_fds_copy, NULL, NULL);
#endif /* USE_KQUEUE */
if (cc < 0 && !SOFT_ERROR(errno)) {
isc__strerror(errno, strbuf, sizeof(strbuf));
FATAL_ERROR(__FILE__, __LINE__,
"%s %s: %s", fnname,
isc_msgcat_get(isc_msgcat,
ISC_MSGSET_GENERAL,
ISC_MSG_FAILED,
"failed"), strbuf);
}
#if defined(USE_DEVPOLL) && defined(ISC_SOCKET_USE_POLLWATCH)
if (cc == 0) {
if (pollstate == poll_active)
pollstate = poll_checking;
else if (pollstate == poll_checking)
pollstate = poll_idle;
} else if (cc > 0) {
if (pollstate == poll_checking) {
/*
* XXX: We‘d like to use a more
* verbose log level as it‘s actually an
* unexpected event, but the kernel bug
* reportedly happens pretty frequently
* (and it can also be a false positive)
* so it would be just too noisy.
*/
manager_log(manager,
ISC_LOGCATEGORY_GENERAL,
ISC_LOGMODULE_SOCKET,
ISC_LOG_DEBUG(1),
"unexpected POLL timeout");
}
pollstate = poll_active;
}
#endif
} while (cc < 0);
#if defined(USE_KQUEUE) || defined (USE_EPOLL) || defined (USE_DEVPOLL)
done = process_fds(manager, manager->events, cc);
#elif defined(USE_SELECT)
process_fds(manager, maxfd, manager->read_fds_copy,
manager->write_fds_copy);
/*
* Process reads on internal, control fd.
*/
if (FD_ISSET(ctlfd, manager->read_fds_copy))
done = process_ctlfd(manager);
#endif
}
manager_log(manager, TRACE, "%s",
isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
ISC_MSG_EXITING, "watcher exiting"));
return ((isc_threadresult_t)0);
}
无限循环，epoll_wait当监听的epoll_fd队列上有IO事件发生时，将对应的socket fd和事件放入events数组中，并且将这些注册在epoll_fd上的socket fd对应事件清空。
process_fds遍历数组，找到对应的socket fd，并判断该fd是不是线程控制管道，如果是则会在执行完其他socket fd上的对应事件后再处理管道中的控制消息。
static isc_boolean_t
process_fds(isc__socketmgr_t *manager, struct epoll_event *events, int nevents)
{
int i;
isc_boolean_t done = ISC_FALSE;
#ifdef USE_WATCHER_THREAD
isc_boolean_t have_ctlevent = ISC_FALSE;
#endif
if (nevents == manager->nevents) {
manager_log(manager, ISC_LOGCATEGORY_GENERAL,
ISC_LOGMODULE_SOCKET, ISC_LOG_INFO,
"maximum number of FD events (%d) received",
nevents);
}
for (i = 0; i < nevents; i++) {
REQUIRE(events[i].data.fd < (int)manager->maxsocks);
#ifdef USE_WATCHER_THREAD
if (events[i].data.fd == manager->pipe_fds[0]) {
have_ctlevent = ISC_TRUE;
continue;
}
#endif
if ((events[i].events & EPOLLERR) != 0 ||
(events[i].events & EPOLLHUP) != 0) {
/*
* epoll does not set IN/OUT bits on an erroneous
* condition, so we need to try both anyway. This is a
* bit inefficient, but should be okay for such rare
* events. Note also that the read or write attempt
* won‘t block because we use non-blocking sockets.
*/
events[i].events |= (EPOLLIN | EPOLLOUT);
}
process_fd(manager, events[i].data.fd,
(events[i].events & EPOLLIN) != 0,
(events[i].events & EPOLLOUT) != 0);
}
#ifdef USE_WATCHER_THREAD
if (have_ctlevent)
done = process_ctlfd(manager);
#endif
return (done);
}
待续
时间： 2024-08-29 12:28:56