epoll实现源码分析
0, 说明
很久以前就分析了，没有写成文章。最近又复习了一下epoll的实现，分析如下：
1, 数据结构
poll的安全唤醒队列，以及spinlock锁结构。
/*
* This is used to implement the safe poll wake up avoiding to reenter
* the poll callback from inside wake_up().
*/
struct poll_safewake {
struct list_head wake_task_list;
spinlock_t lock;
};
. epoll 文件结构
struct epoll_filefd {
struct file *file;
int fd;
};
. eventpoll 结构
这是一个实现epoll时，内部实现使用的数据结构。
在epoll_create中会为epoll本身获取一个和文件描述符和一个file结构，file->priv_data就是指向eventpoll结构。
该结构在函数：epoll_create()->sys_epoll_create1()中创建。
并在后续的epoll操作中都会用到。
/*
* This structure is stored inside the "private_data" member of the file
* structure and rapresent the main data sructure for the eventpoll
* interface.
*/
struct eventpoll {
/* Protect the this structure access */
spinlock_t lock; //保护本结构的自旋锁
/*
* This mutex is used to ensure that files are not removed
* while epoll is using them. This is held during the event
* collection loop, the file cleanup path, the epoll file exit
* code and the ctl operations.
*/
struct mutex mtx;
/* Wait queue used by sys_epoll_wait() */
wait_queue_head_t wq; // 等待队列头结点，sys_epoll_wait()使用
/* Wait queue used by file->poll() */
wait_queue_head_t poll_wait; // file->poll()使用的等待队列头
/* List of ready file descriptors */
struct list_head rdllist;
/* RB tree root used to store monitored fd structs */
struct rb_root rbr; // 保存监控fd结构的红黑树
/*
* This is a single linked list that chains all the "struct epitem" that
* happened while transfering ready events to userspace w/out
* holding ->lock.
*/
struct epitem *ovflist; // 单链表，链接所有发生的准备复制到用户空间的事件
};
.epitem结构
每一个添加到eventpoll接口的文件描述符都对应一个epitem类型的实体，该结构中用rbn连接到eventpoll结构的红黑树根变量rbr。
/*
* Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree.
*/
struct epitem {
/* RB tree node used to link this structure to the eventpoll RB tree */
struct rb_node rbn;
/* List header used to link this structure to the eventpoll ready list */
struct list_head rdllink;
/*
* Works together "struct eventpoll"->ovflist in keeping the
* single linked chain of items.
*/
struct epitem *next;
/* The file descriptor information this item refers to */
struct epoll_filefd ffd;
/* Number of active wait queue attached to poll operations */
int nwait;
/* List containing poll wait queues */
struct list_head pwqlist;
/* The "container" of this item */
struct eventpoll *ep; // 该epitem属于ep指向的eventpoll结构
/* List header used to link this item to the "struct file" items list */
struct list_head fllink;
/* The structure that describe the interested events and the source fd */
struct epoll_event event;
};
. eppoll_entry 结构
/* Wait structure used by the poll hooks */
struct eppoll_entry {
/* List header used to link this structure to the "struct epitem" */
struct list_head llink;
/* The "base" pointer is set to the container "struct epitem" */
void *base;
/*
* Wait queue item that will be linked to the target file wait
* queue head.
*/
wait_queue_t wait;
/* The wait queue head that linked the "wait" wait queue item */
wait_queue_head_t *whead;
};
. epoll_event结构
struct epoll_event {
__u32 events;
__u64 data;
} EPOLL_PACKED;
. epoll队列
/* Wrapper struct used by poll queueing */
struct ep_pqueue {
poll_table pt;
struct epitem *epi;
};
/*
* structures and helpers for f_op->poll implementations
*/
typedef void (*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_table_struct *);
typedef struct poll_table_struct {
poll_queue_proc qproc;
} poll_table;
. 结构体之间关系
2，实现解析
2.1 初始化
epoll系统调用以模块的方式添加到内核，整个系统调用的实现是在文件Eventpoll.c文件中。epoll的初始化流程如下：
* epoll模块初始化
epoll模块初始化函数主要对epoll用到的全局锁进行初始化，同时创建epitem结构和eppoll_entry结构内存池。
static int __init eventpoll_init(void)
{
mutex_init(&epmutex);
/* Initialize the structure used to perform safe poll wait head wake ups */
// 初始化静态全局变量psw锁和队列， 该变量在模块加载时被定义：
// /* Safe wake up implementation */
// static struct poll_safewake psw;
ep_poll_safewake_init(&psw);
/* Allocates slab cache used to allocate "struct epitem" items */
// epitem的内存池
epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
0, SLAB_HWCACHE_ALIGN|EPI_SLAB_DEBUG|SLAB_PANIC,
NULL);
/* Allocates slab cache used to allocate "struct eppoll_entry" */
// 为eppoll_entry结构创建内存池
pwq_cache = kmem_cache_create("eventpoll_pwq",
sizeof(struct eppoll_entry), 0,
EPI_SLAB_DEBUG|SLAB_PANIC, NULL);
return 0;
}
fs_initcall(eventpoll_init);
2.2 epoll_create()的实现
---------------------------------------------------
#include
int epoll_create(int size)
----------------------------------------------------
epoll使用内核的通用文件系统架构来实现的，所以，就需要注册一个struct file_operations的结构，并实现其中对应的回调函数。
// epoll文件系统中的file结构中f_op的值，也就是file->f_op的指针值
/* File callbacks that implement the eventpoll file behaviour */
static const struct file_operations eventpoll_fops = {
.release = ep_eventpoll_release,
.poll = ep_eventpoll_poll
};
// epoll_create(int size) 参数size其实没有用到，大于0即可。
asmlinkage long sys_epoll_create(int size)
{
// 若size小于0，返回值错误编号
if (size < 0)
return -EINVAL;
// 这里的size参数其实没有用，只要大于0，就可以了。
// 这里根本就没有使用size，而是写成了0。分析的版本是2.6.27，可能在以后的版本中会有时改变。
return sys_epoll_create1(0);
}
// 最终的epoll_create是在下面的函数中实现的
/*
* Open an eventpoll file descriptor.
*/
asmlinkage long sys_epoll_create1(int flags)
{
int error, fd = -1;
// 该结构将会被赋值给file结构的priv_data指针
struct eventpoll *ep;
/* Check the EPOLL_* constant for consistency. */
BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
if (flags & ~EPOLL_CLOEXEC)
return -EINVAL;
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d)\n",
current, flags));
/*
* Create the internal data structure ( "struct eventpoll" ).
*/
// 创建一个eventpoll结构，并对该结构中的成员进行初始化
// 之后该eventpoll结构成员的指针，将会赋给file->private_data字段
error = ep_alloc(&ep);
if (error < 0) {
fd = error;
goto error_return;
}
/*
* Creates all the items needed to setup an eventpoll file. That is,
* a file structure and a free file descriptor.
*/
// 在匿名文件系统(anon_inode_mnt->mnt_sb->s_root)的超级块根路径下创建一个目录结构(d_entry)
// 在该目录结构下，申请一个fd和file结构，并把这两者连接起来。
// 初始化file结构的private_data指针的值为ep(eventpoll *参数)的指针。
// 初始化file的f_op指针，为eventpoll_fops结构的指针，该结构的定义在本节开始处。
fd = anon_inode_getfd("[eventpoll]", &eventpoll_fops, ep,
flags & O_CLOEXEC);
if (fd < 0)
ep_free(ep);
error_return:
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %d\n",
current, flags, fd));
// 返回epoll创建的fd
return fd;
}
// 创建并初始化eventpoll结构
static int ep_alloc(struct eventpoll **pep)
{
struct eventpoll *ep = kzalloc(sizeof(*ep), GFP_KERNEL);
if (!ep)
return -ENOMEM;
spin_lock_init(&ep->lock);
mutex_init(&ep->mtx);
init_waitqueue_head(&ep->wq);
init_waitqueue_head(&ep->poll_wait);
INIT_LIST_HEAD(&ep->rdllist);
ep->rbr = RB_ROOT;
ep->ovflist = EP_UNACTIVE_PTR;
*pep = ep;
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_alloc() ep=%p\n",
current, ep));
return 0;
}
// epoll的file_operation的poll函数定义
static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait)
{
unsigned int pollflags = 0;
unsigned long flags;
struct eventpoll *ep = file->private_data;
/* Insert inside our poll wait queue */
poll_wait(file, &ep->poll_wait, wait);
/* Check our condition */
spin_lock_irqsave(&ep->lock, flags);
if (!list_empty(&ep->rdllist))
pollflags = POLLIN | POLLRDNORM;
spin_unlock_irqrestore(&ep->lock, flags);
return pollflags;
}
小结：epoll_create的要点如下：
(1) epoll的整个实现建立在内核的文件系统框架基础之上。
(2) epoll向内核文件系统注册了一个file_operations结构，并实现其中的poll和release回调函数。
(3) epoll文件系统中,poll回调函数的实现函数是：ep_eventpoll_poll，release回调函数是：ep_eventpoll_release。
(4) epoll_create(int size) 其中的size参数在内核中实际上没有使用。
(5) epoll_create函数实现初始化操作，在匿名文件系统上注册了一个目录结构和其操作。
2.3 epoll控制函数的实现(epoll_ctl)
通过该函数对epoll监控的fd进行添加，删除，修改操作。
/*
* The following function implements the controller interface for
* the eventpoll file that enables the insertion/removal/change of
* file descriptors inside the interest set.
*/
asmlinkage long sys_epoll_ctl(int epfd, int op, int fd,
struct epoll_event __user *event)
{
int error;
struct file *file, *tfile;
struct eventpoll *ep;
struct epitem *epi;
struct epoll_event epds;
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_ctl(%d, %d, %d, %p)\n",
current, epfd, op, fd, event));
error = -EFAULT;
if (ep_op_has_event(op) &&
copy_from_user(&epds, event, sizeof(struct epoll_event)))
goto error_return;
/* Get the "struct file *" for the eventpoll file */
error = -EBADF;
// 获取epoll的file结构指针
file = fget(epfd);
if (!file)
goto error_return;
/* Get the "struct file *" for the target file */
// 获取被监控fd的file结构指针
tfile = fget(fd);
if (!tfile)
goto error_fput;
/* The target file descriptor must support poll */
error = -EPERM;
// 若被监控的fd对应的file结构没有poll函数，错误
// 因为后续需要调用对应文件系统的poll函数，所以这里要进行判断。
// 对于tcp的socket来说，这里是tcp_poll，对于udp的socket来说，是udp_poll
if (!tfile->f_op || !tfile->f_op->poll)
goto error_tgt_fput;
/*
* We have to check that the file structure underneath the file descriptor
* the user passed to us _is_ an eventpoll file. And also we do not permit
* adding an epoll file descriptor inside itself.
*/
error = -EINVAL;
// 若被监控的file结构指针和epoll的file结构指针相等，则把两个file都销毁，并返回错误。
if (file == tfile || !is_file_epoll(file))
goto error_tgt_fput;
/*
* At this point it is safe to assume that the "private_data" contains
* our own data structure.
*/
// file->private_data是eventpoll结构，前面已经讲过，这里要获取该指针
ep = file->private_data;
// 添加互斥锁
mutex_lock(&ep->mtx);
/*
* Try to lookup the file inside our RB tree, Since we grabbed "mtx"
* above, we can be sure to be able to use the item looked up by
* ep_find() till we release the mutex.
*/
// 在ep的红黑树中，查找添加的fd是否存在。
// 若已经存在，直接返回epitem的地址，否则返回NULL。
epi = ep_find(ep, tfile, fd);
error = -EINVAL;
switch (op) {
case EPOLL_CTL_ADD: // 是插入监控fd操作
// ep对应的epitem红黑树中不存在该fd
if (!epi) {
// 默认要添加POLLERR和POLLHUP事件
epds.events |= POLLERR | POLLHUP;
// 把该file和fd插入到ep对应的epitem红黑树中。
error = ep_insert(ep, &epds, tfile, fd);
} else //若插入的fd已存在，直接返回EXIST错误码
error = -EEXIST;
break;
case EPOLL_CTL_DEL: //删除fd操作
if (epi)
error = ep_remove(ep, epi); //从ep的红黑树中删除fd
else
error = -ENOENT;
break;
case EPOLL_CTL_MOD: //修改fd操作
if (epi) {
// 需要修改fd的关注的事件，默认会添加两个事件，一个是ERR事件，一个是HUP事件。
epds.events |= POLLERR | POLLHUP;
error = ep_modify(ep, epi, &epds);
} else
error = -ENOENT;
break;
}
// 解锁
mutex_unlock(&ep->mtx);
error_tgt_fput:
fput(tfile);
error_fput:
fput(file);
error_return:
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_ctl(%d, %d, %d, %p) = %d\n",
current, epfd, op, fd, event, error));
return error;
}
. 添加监控fd（ep_insert函数）
/*
* Must be called with "mtx" held.
*/
static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
struct file *tfile, int fd)
{
int error, revents, pwake = 0;
unsigned long flags;
struct epitem *epi;
struct ep_pqueue epq;
error = -ENOMEM;
if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
goto error_return;
/* Item initialization follow here ... */
INIT_LIST_HEAD(&epi->rdllink);
INIT_LIST_HEAD(&epi->fllink);
INIT_LIST_HEAD(&epi->pwqlist);
epi->ep = ep;
ep_set_ffd(&epi->ffd, tfile, fd);
epi->event = *event;
epi->nwait = 0;
epi->next = EP_UNACTIVE_PTR;
/* Initialize the poll table using the queue callback */
epq.epi = epi;
// 为epq.pt安装处理函数，pt->qproc = ep_ptable_queue_proc；
// 这里实际上是安装了一个回调函数，该回调函数会在fd对应文件系统的poll函数中被调用。
// 对于socket的fd来说，f_op->poll回调函数是sock_poll()，最终调用sock协议相关的函数，
// 若是TCP的socket就是调用tcp_poll()，函数，在该函数中会调用 poll_wait(file, sk->sk_sleep, wait);
// 而poll_wait()函数会调用p->qproc(filp, wait_address, p);在这里也就是ep_ptable_queue_proc()。
init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
/*
* Attach the item to the poll hooks and get current event bits.
* We can safely use the file* here because its usage count has
* been increased by the caller of this function. Note that after
* this operation completes, the poll callback can start hitting
* the new item.
*/
// 调用fd对应文件系统的poll函数
// 对于socket来说，f_op->poll()函数是：sock_poll(struct file *file, poll_table *wait)函数
// 在sock_poll()中的调用顺序为：sock_poll()->file->private_data->ops->poll()也就是调用tcp_poll()
revents = tfile->f_op->poll(tfile, &epq.pt);
/*
* We have to check if something went wrong during the poll wait queue
* install process. Namely an allocation for a wait queue failed due
* high memory pressure.
*/
if (epi->nwait < 0)
goto error_unregister;
/* Add the current item to the list of active epoll hook for this file */
spin_lock(&tfile->f_ep_lock);
list_add_tail(&epi->fllink, &tfile->f_ep_links);
spin_unlock(&tfile->f_ep_lock);
/*
* Add the current item to the RB tree. All RB tree operations are
* protected by "mtx", and ep_insert() is called with "mtx" held.
*/
ep_rbtree_insert(ep, epi);
/* We have to drop the new item inside our item list to keep track of it */
spin_lock_irqsave(&ep->lock, flags);
/* If the file is already "ready" we drop it inside the ready list */
if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
/* Notify waiting tasks that events are available */
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
if (waitqueue_active(&ep->poll_wait))
pwake++;
}
spin_unlock_irqrestore(&ep->lock, flags);
/* We have to call this outside the lock */
if (pwake)
ep_poll_safewake(&psw, &ep->poll_wait);
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_insert(%p, %p, %d)\n",
current, ep, tfile, fd));
return 0;
error_unregister:
ep_unregister_pollwait(ep, epi);
/*
* We need to do this because an event could have been arrived on some
* allocated wait queue. Note that we don't care about the ep->ovflist
* list, since that is used/cleaned only inside a section bound by "mtx".
* And ep_insert() is called with "mtx" held.
*/
spin_lock_irqsave(&ep->lock, flags);
if (ep_is_linked(&epi->rdllink))
list_del_init(&epi->rdllink);
spin_unlock_irqrestore(&ep->lock, flags);
kmem_cache_free(epi_cache, epi);
error_return:
return error;
}
. ep_modify() 函数的实现
当epoll_ctl()函数的操作是EPOLL_CTL_MOD时，会调用ep_modify()函数改变要监控fd的事件。
/*
* Modify the interest event mask by dropping an event if the new mask
* has a match in the current file status. Must be called with "mtx" held.
*/
static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_event *event)
{
int pwake = 0;
unsigned int revents;
unsigned long flags;
/*
* Set the new event interest mask before calling f_op->poll(), otherwise
* a potential race might occur. In fact if we do this operation inside
* the lock, an event might happen between the f_op->poll() call and the
* new event set registering.
*/
epi->event.events = event->events;
/*
* Get current event bits. We can safely use the file* here because
* its usage count has been increased by the caller of this function.
*/
// 当poll_table的参数为NULL时，对应文件系统注册的poll函数将不再执行回调函数ep_ptable_queue_proc()，
// 因为该回调函数已经注册过了。这里只需要直接返回准备好的事件对应的位。
revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL);
// 添加自旋锁
spin_lock_irqsave(&ep->lock, flags);
/* Copy the data member from inside the lock */
// 把用户要监控的事件，复制给eventpoll句柄中的事件变量
epi->event.data = event->data;
/*
* If the item is "hot" and it is not registered inside the ready
* list, push it inside.
*/
// 若在用户需要监控的事件中，有已经准备好的事件，把该事件添加到已准备好fd队列中，然后唤醒等待队列。
if (revents & event->events) {
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
/* Notify waiting tasks that events are available */
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
if (waitqueue_active(&ep->poll_wait))
pwake++;
}
}
spin_unlock_irqrestore(&ep->lock, flags); // 解锁
/* We have to call this outside the lock */
// 遍历等待队列，并执行对应的回调函数，该回调函数把准备好的fd，添加到已完成fd队列中
if (pwake)
ep_poll_safewake(&psw, &ep->poll_wait);
return 0;
}
. poll的回调函数
//这里的参数：whead是对应文件系统的等待队列头节点，例如：TCPsocket的就是sk->sk_sleep，而这里的pt，就是epoll文件系统中的ep_pqueue结构中的poll队列。
// 该函数创建一个eppoll_entry 实体，初始化该结构，为wait队列添加一个回调函数ep_poll_callback
// 并把该结构实体添加到，文件系统对应的poll等待队列中。
/*
* This is the callback that is used to add our wait queue to the
* target file wakeup lists.
*/
static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
poll_table *pt)
{
// 从pt结构回指指针中获取epitem结构指针。
struct epitem *epi = ep_item_from_epqueue(pt);
struct eppoll_entry *pwq;
// 创建一个eppoll_entry 结构，并把该结构的wait实体添加到对应文件系统的等待队列中。
if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
// 为新创建的eppoll_entry 实体设置等待队列唤醒的回调函数ep_poll_callback。
init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
// 设置对应文件系统的等待队列头指针
pwq->whead = whead;
pwq->base = epi;
// 把新建立的ep_poll_callback实体的等待队列节点wait，添加到对应文件系统的等待队列中。
add_wait_queue(whead, &pwq->wait);
// 把创建的新的eppoll_entry 结构体，添加到epollevent建立的eppoll_entry 结构队列中。
list_add_tail(&pwq->llink, &epi->pwqlist);
epi->nwait++;
} else {
/* We have to signal that an error occurred */
// 有错发生，比如：内存不足等。
epi->nwait = -1;
}
}
/*
* This is the callback that is passed to the wait queue wakeup
* machanism. It is called by the stored file descriptors when they
* have events to report.
*/
// epoll之所以能这么高效，是靠这个回调函数来把有事件的描述符添加到epoll准备好的事件队列中的。
// 该函数是epoll的通知引擎，实现其实很简单。
static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
int pwake = 0;
unsigned long flags;
struct epitem *epi = ep_item_from_wait(wait);
struct eventpoll *ep = epi->ep;
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: poll_callback(%p) epi=%p ep=%p\n",
current, epi->ffd.file, epi, ep));
spin_lock_irqsave(&ep->lock, flags);
/*
* If the event mask does not contain any poll(2) event, we consider the
* descriptor to be disabled. This condition is likely the effect of the
* EPOLLONESHOT bit that disables the descriptor when an event is received,
* until the next EPOLL_CTL_MOD will be issued.
*/
//
if (!(epi->event.events & ~EP_PRIVATE_BITS))
goto out_unlock;
/*
* If we are trasfering events to userspace, we can hold no locks
* (because we're accessing user memory, and because of linux f_op->poll()
* semantics). All the events that happens during that period of time are
* chained in ep->ovflist and requeued later on.
*/
if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
if (epi->next == EP_UNACTIVE_PTR) {
epi->next = ep->ovflist;
ep->ovflist = epi;
}
goto out_unlock;
}
/* If this file is already in the ready list we exit soon */
// 若file结构已经添加到了准备队列，什么都不做
if (ep_is_linked(&epi->rdllink))
goto is_linked;
// 把epi节点添加到，eventepoll的已准备好的队列中。
list_add_tail(&epi->rdllink, &ep->rdllist);
is_linked:
/*
* Wake up ( if active ) both the eventpoll wait list and the ->poll()
* wait list.
*/
// 唤醒epoll的等待队列
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
if (waitqueue_active(&ep->poll_wait))
pwake++;
out_unlock:
spin_unlock_irqrestore(&ep->lock, flags);
/* We have to call this outside the lock */
if (pwake)
ep_poll_safewake(&psw, &ep->poll_wait);
return 1;
}
总结要点：
(1) epoll是基于linux虚拟文件系统框架实现的。
(2) 在epoll_ctl函数中，其实就是往epoll_create创建的eventpoll变量中的epitem红黑树中插入被监控fd和file的过程。
(3) 在插入过程中，设置poll_table参数中的回调函数qproc为ep_ptable_queue_proc。然后调用被监控fd对应文件系统的poll函数，tcp的是tcp_poll,udp的是udp_poll。在这些poll函数中，会把poll_table传入作为参数，而且会调用poll_table->qproc()函数，也就是这里的ep_ptable_queue_proc。
(4) 在ep_ptable_queue_proc函数中，会为被监控fd设置中断唤醒回调函数ep_poll_callback。当该fd有事件发生，而进程被唤醒时，则会调用ep_poll_callback函数。
(5) ep_poll_callback函数主要功能是把fd对应的epitem添加到evenpoll的rdllink队列中，表示该fd有事件发生了。
(6) 而epoll_wait函数，最终会把rdllink队列的数据，往用户层复制，告诉用户那些fd发生了什么事件。
