select, poll, epoll都是Linux上的IO多路复用机制．知其然知其所以然，为了更好地理解其底层实现，这几天我阅读了这三个系统调用的源码．
以下源代码摘自Linux4.4.0内核．
预备知识
在了解IO多路复用技术之前，首先需要了解Linux内核的3个方面．
1.等待队列waitqueue
等待队列(@ include/linux/wait.h)的队列头(wait_queue_head_t)往往是资源生产者，队列成员(wait_queue_t)往往是资源消费者．当队列头的资源ready后，内核会逐个执行每个队列成员指定的回调函数，来通知它们该资源已经ready了．
2.内核的poll机制
被poll的fd，必须在实现上支持内核的poll技术，比如fd是某个字符设备，或者是一个socket，它必须实现file_operation中的poll操作，这个操作会给该fd分配一个等待队列头．主动poll该fd的进程必须分配一个等待队列成员，并将其添加到该fd的等待队列中去，同时指定该资源ready时的回调函数．拿socket举例，它必须实现一个poll操作，该操作是由发起轮询者(即监听它的进程)主动调用的．这个poll操作必须调用poll_wait()，后者会将发起轮询者作为等待队列成员添加到该socket的等待队列中．当该socket发生状态改变时，就会通过队列头逐个通知所有监听它的进程．
3.epollfd
epollfd其实质就是一个fd．
select
1.源码分析
// @ fs/select.c
SYSCALL_DEFINE5(select, int, n, fd_set __user *, inp, fd_set __user *, outp,
fd_set __user *, exp, struct timeval __user *, tvp)
{
struct timespec end_time, *to = NULL;
struct timeval tv;
int ret;
if (tvp) { // 如果timeout值不为NULL
if (copy_from_user(&tv, tvp, sizeof(tv))) // 则把超时值从用户空间拷贝到内核空间．
return -EFAULT;
to = &end_time;
// 计算timespec格式的未来timeout时间．
if (poll_select_set_timeout(to,
tv.tv_sec + (tv.tv_usec / USEC_PER_SEC),
(tv.tv_usec % USEC_PER_SEC) * NSEC_PER_USEC))
return -EINVAL;
}
ret = core_sys_select(n, inp, outp, exp, to); // 关键函数
// poll_select_copy_remaining函数的作用是：
// 如果有超时值，则把距离超时时间所剩的时间，从内核空间拷贝到用户空间．
ret = poll_select_copy_remaining(&end_time, tvp, 1, ret);
return ret;
}
int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
fd_set __user *exp, struct timespec *end_time)
{
fd_set_bits fds;
/**
@ include/linux/poll.h
typedef struct {
unsigned long *in, *out, *ex;
unsigned long *res_in, *res_out, *res_ex;
} fd_set_bits;
**/
// 该结构体内部定义的都是指针，指向描述符集合．
void *bits;
int ret, max_fds;
unsigned int size;
struct fdtable *fdt;
/**
@ include/linux/fdtable.h
struct fdtable {
unsigned int max_fds;
struct file __rcu **fd; // current fd array
unsigned long *close_on_exec;
unsigned long *open_fds;
unsigned long *full_fds_bits;
struct rcu_head rcu;
};
**/
/* Allocate small arguments on the stack to save memory and be faster */
long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
/**
@ include/linux/poll.h
#define FRONTEND_STACK_ALLOC 256
#define SELECT_STACK_ALLOC FRONTEND_STACK_ALLOC
**/
// 256 / 8 = 32
// 预先在stack中分配小部分的空间，以节省内存且更加快速
ret = -EINVAL;
if (n < 0)
goto out_nofds;
/* max_fds can increase, so grab it once to avoid race */
rcu_read_lock();
fdt = files_fdtable(current->files); // 获取当前进程的文件描述符表
max_fds = fdt->max_fds;
rcu_read_unlock();
if (n > max_fds) // 如果传入的n大于当前进程最大的文件描述符，则修改之．
n = max_fds;
/*
* We need 6 bitmaps (in/out/ex for both incoming and outgoing),
* since we used fdset we need to allocate memory in units of
* long-words.
*/
/**
需要分配6个bitmap,
用户传入的in, out, ex,
以及返回给用户的res_in, res_out, res_ex.
**/
size = FDS_BYTES(n); // 以一个描述符占一个bit来计算，求出传入的描述符需要多少个byte(以long为单位分配byte)。
bits = stack_fds;
if (size > sizeof(stack_fds) / 6) {
// 因为一共有６个bitmap, 所以除以６，求得每个bitmap所占的byte个数，用以和size比较。
/* Not enough space in on-stack array; must use kmalloc */
ret = -ENOMEM;
bits = kmalloc(6 * size, GFP_KERNEL); // 如果预先在stack分配的空间太小，则在heap上申请内存。
if (!bits)
goto out_nofds;
}
// fds结构体的妙用
fds.in = bits;
fds.out = bits + size;
fds.ex = bits + 2*size;
fds.res_in = bits + 3*size;
fds.res_out = bits + 4*size;
fds.res_ex = bits + 5*size;
/**
@ include/linux/poll.h
static inline
int get_fd_set(unsigned long nr, void __user *ufdset, unsigned long *fdset)
{
nr = FDS_BYTES(nr);
if (ufdset)
return copy_from_user(fdset, ufdset, nr) ? -EFAULT : 0;
memset(fdset, 0, nr);
return 0;
}
get_fd_set的作用是调用copy_from_user函数，
把用户传入的fd_set从用户空间拷贝到内核空间。
**/
if ((ret = get_fd_set(n, inp, fds.in)) ||
(ret = get_fd_set(n, outp, fds.out)) ||
(ret = get_fd_set(n, exp, fds.ex)))
goto out;
// 对返回给用户的fd_set初始化为零
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);
ret = do_select(n, &fds, end_time); // 关键函数，完成主要的工作。
if (ret < 0) // 出错
goto out;
if (!ret) { // 无文件设备就绪，但超时或有未决信号。
ret = -ERESTARTNOHAND;
if (signal_pending(current))
goto out;
ret = 0;
}
// 把结果集合拷贝到用户空间．
if (set_fd_set(n, inp, fds.res_in) ||
set_fd_set(n, outp, fds.res_out) ||
set_fd_set(n, exp, fds.res_ex))
ret = -EFAULT;
out:
if (bits != stack_fds) // 如果申请了heap上的内存，则释放之．
kfree(bits);
out_nofds:
return ret;
}
int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
{
ktime_t expire, *to = NULL;
struct poll_wqueues table;
/**
@ inclue/linux/poll.h
struct poll_wqueues {
poll_table pt;
struct poll_table_page *table;
struct task_struct *polling_task;
int triggered;
int error;
int inline_index;
struct poll_table_entry inline_entries[N_INLINE_POLL_ENTRIES];
};
**/
poll_table *wait;
/**
@ inclue/linux/poll.h
typedef void (*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_table_struct *);
typedef struct poll_table_struct {
poll_queue_proc _qproc;
unsigned long _key;
} poll_table;
**/
int retval, i, timed_out = 0;
unsigned long slack = 0;
unsigned int busy_flag = net_busy_loop_on() ? POLL_BUSY_LOOP : 0; // TODO
unsigned long busy_end = 0;
rcu_read_lock(); // TODO
retval = max_select_fd(n, fds); // 检查fds中fd的有效性(即要求打开)，并获得当前最大的fd．
rcu_read_unlock();
if (retval < 0)
return retval;
n = retval;
// 初始化poll_wqueues对象table，
// 包括初始化poll_wqueues.poll_table._qproc函数指针为__pollwait．
poll_initwait(&table);
/**
@ fs/select.c
void poll_initwait(struct poll_wqueues *pwq)
{
init_poll_funcptr(&pwq->pt, __pollwait);
pwq->polling_task = current;
pwq->triggered = 0;
pwq->error = 0;
pwq->table = NULL;
pwq->inline_index = 0;
}
**/
wait = &table.pt;
// 如果用户传入的timeout不为NULL，但设定的时间为0，
// 则设置poll_table._qproc函数指针为NULL，
// 并设置timed_out = 1．
if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
wait->_qproc = NULL;
timed_out = 1;
}
// 如果用户传入的timeout不为NULL，且设定的timeout时间不为0，
// 则转换timeout时间．
if (end_time && !timed_out)
slack = select_estimate_accuracy(end_time);
retval = 0;
for (;;) {
unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;
bool can_busy_loop = false;
inp = fds->in; outp = fds->out; exp = fds->ex;
rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;
for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
unsigned long in, out, ex, all_bits, bit = 1, mask, j;
unsigned long res_in = 0, res_out = 0, res_ex = 0;
// 将in, out, exception进行位或运算，得到all_bits．
in = *inp++; out = *outp++; ex = *exp++;
all_bits = in | out | ex;
// 如果这BITS_PER_LONG个描述符不需要被监听，
// 则continue到下一个32个描述符中的循环．
if (all_bits == 0) {
i += BITS_PER_LONG;
continue;
}
// 本次这BITS_PER_LONG个描述符中有需要监听的．
for (j = 0; j < BITS_PER_LONG; ++j, ++i, bit <<= 1) {
struct fd f;
if (i >= n) // 检测i是否超出了待监听的最大描述符．
break;
// 每次循环后bit左移一位，用来跳过不需要被监听的描述符．
if (!(bit & all_bits))
continue;
f = fdget(i); // 获取file结构，并增加其引用计数．
if (f.file) {
const struct file_operations *f_op;
f_op = f.file->f_op;
mask = DEFAULT_POLLMASK;
if (f_op->poll) {
wait_key_set(wait, in, out, // 设置当前描述符待监听的事件掩码．
bit, busy_flag);
mask = (*f_op->poll)(f.file, wait);
// f_op->poll函数执行如下：
// 1, 调用poll_wait函数(在include/linux/poll.h)；
// 2, 检测当前描述符所对应的文件设备的状态，并返回状态掩码mask．
// poll_wait函数的定义如下：
/**
@ include/linux/poll.h
static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
if (p && p->_qproc && wait_address)
p->_qproc(filp, wait_address, p);
}
**/
// 而由上文的poll_initwait函数可知，p->_qproc指向__pollwait函数．
// __pollwait函数的定义如下：
/**
@ fs/select.c
// __pollwait函数的作用是把当前进程添加到当前文件设备的等待队列中．
static void __pollwait(struct file *filp, wait_queue_head_t *wait_address, poll_table *p)
{
struct poll_wqueues *pwq = container_of(p, struct poll_wqueues, pt);
struct poll_table_entry *entry = poll_get_entry(pwq);
if (!entry)
return;
entry->filp = get_file(filp);
entry->wait_address = wait_address;
entry->key = p->_key;
init_waitqueue_func_entry(&entry->wait, pollwake);
entry->wait.private = pwq;
add_wait_queue(wait_address, &entry->wait);
}
**/
}
fdput(f); // 释放file结构指针，实质是减少其引用计数．
// mask是执行f_op->poll函数后所返回的文件设备状态掩码．
if ((mask & POLLIN_SET) && (in & bit)) {
res_in |= bit; // 描述符对应的文件设备可读
retval++;
wait->_qproc = NULL; // 避免重复执行__pollwait函数
}
if ((mask & POLLOUT_SET) && (out & bit)) {
res_out |= bit; // 描述符对应的文件设备可写
retval++;
wait->_qproc = NULL;
}
if ((mask & POLLEX_SET) && (ex & bit)) {
res_ex |= bit; // 描述符对应的文件设备发生error
retval++;
wait->_qproc = NULL;
}
/* got something, stop busy polling */
if (retval) {
can_busy_loop = false;
busy_flag = 0;
/*
* only remember a returned
* POLL_BUSY_LOOP if we asked for it
*/
} else if (busy_flag & mask)
can_busy_loop = true;
}
}
// 根据f_op->poll函数的结果，设置rinp, routp, rexp
if (res_in)
*rinp = res_in;
if (res_out)
*routp = res_out;
if (res_ex)
*rexp = res_ex;
cond_resched();
}
wait->_qproc = NULL; // 避免重复执行__pollwait函数
// 如果有文件设备就绪，或超时，或有未决信号；
// 亦或者发生错误，
// 则退出大循环．
if (retval || timed_out || signal_pending(current))
break;
if (table.error) {
retval = table.error;
break;
}
/* only if found POLL_BUSY_LOOP sockets && not out of time */
if (can_busy_loop && !need_resched()) { // TODO
if (!busy_end) {
busy_end = busy_loop_end_time();
continue;
}
if (!busy_loop_timeout(busy_end))
continue;
}
busy_flag = 0;
/*
* If this is the first loop and we have a timeout
* given, then we convert to ktime_t and set the to
* pointer to the expiry value.
*/
if (end_time && !to) {
expire = timespec_to_ktime(*end_time); // 转换timeout时间．
to = &expire;
}
// 第一次循环时，当前用户进程在这里进入睡眠．
// 超时，poll_schedule_timeout()返回0；被唤醒时返回-EINTR．
if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,
to, slack))
timed_out = 1;
}
// 把当前进程从所有文件的等待队列中删掉，并回收内存．
poll_freewait(&table);
// 返回就绪的文件描述符的个数．
return retval;
}2.实现过程
一. 如果用户传入的超时值不为NULL，则把用户空间的timeout对象拷贝到内 核空间，并计算timespec格式的超时时间．
二. 调用core_sys_select函数．过程如下：
- 根据传入的maxfds值，计算出保存所有文件描述符需要多少字节(1个字节占1bit)，再根据字节数判断是在栈上还是堆上分配内存(如果字节数太大，则在堆上分配)．一共需要分配6个bitmap，用户传入的in，out，ex，以及返回给用户的res_in，res_out，res_ex．
- 将3个用户传入的fdset(in, out, ex)从用户空间拷贝到内核空间，并将3个返回给用户的fdset(res_in, res_out, res_ex)初始化为0.
- 调用do_select函数．过程如下：
3a. 检查传入的fds中fd的有效性．
3b. 调用poll_initwait函数，以初始化poll_wqueues对象table，其中初始化poll_wqueues.poll_table._qproc函数指针为__pollwait.
3c. 如果传入的超时值不为空，但设定的时间为0，则设置poll_table._qproc函数指针为NULL．
3d. 进入大循环．
3e. 将in, out, ex进行位或运算，得到all_bits．然后遍历all_bits中bit为1的文件描述符，根据当前进程的文件描述符表，获取与当前描述符对应的file结构指针f，并设 置当前描述符待监听的事件掩码，调用f_op->poll函数，该函数会把当前用户进程添加到当前描述符的等待队列中，并获得返回值mask．根据mask设置相应的 返回fdset，执行retval++，并设置wait->_qproc为NULL．
3f. 遍历完所有的文件描述符后，设置wait->_qproc为NULL，并检查是否有文件设备就绪，或超时，或有未决信号，或发生错误，是则退出大循环，执行第8步．
3g. 在第一次循环后，如果设置了超时值，用户进程会调用poll_schedule_timeout进入睡眠．
3h. 最终，调用poll_freewait函数，把当前进程从所有文件的等待队列中删除，回收内存．并返回retval值． - 把res_in, res_out, res_ex从内核空间拷贝到用户空间．并返回ret．
三. 如果设有超时值，则把剩余的超时时间从内核空间拷贝到用户空间．
3.小结
从上述源代码的分析可见，select的低效体现在两个方面：
- 内存复制开销，每次调用select，都要把文件描述符集合从用户空间拷贝到内核空间，又从内核空间拷贝到用户空间．
- 遍历开销，每次调用select，都要在内核中遍历所有传入的文件描述符．
另外select系统调用的第一个参数maxfdp1，其大小被限制在1024内(即FD_SETSIZE)，可监听的文件数量有限，这也是select低效的原因．
poll
poll的实现与select大致相同，故不做赘述．区别有两点：
- 文件描述符集合的结构不同，poll使用的是pollfd，而select使用的是fd_set．
- poll没有限制可监听的文件数量．
epoll
1.源码分析
// @ fs/eventpoll.c
/*
* This structure is stored inside the "private_data" member of the file
* structure and represents the main data structure for the eventpoll
* interface.
*/
// 每创建一个epollfd，内核就会分配一个eventpoll与之对应，可以理解成内核态的epollfd．
struct eventpoll {
/* Protect the access to this structure */
spinlock_t lock;
/*
* 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.
*/
/**
添加，修改或删除监听fd的时候，以及epoll_wait返回，向用户空间传递数据时，都会持有这个互斥锁．
因此，在用户空间中执行epoll相关操作是线程安全的，内核已经做了保护．
**/
struct mutex mtx;
/* Wait queue used by sys_epoll_wait() */
/**
等待队列头部．
当在该等待队列中的进程调用epoll_wait()时，会进入睡眠．
**/
wait_queue_head_t wq;
/* Wait queue used by file->poll() */
/**
用于epollfd被f_op->poll()的时候
**/
wait_queue_head_t poll_wait;
/* List of ready file descriptors */
/**
所有已经ready的epitem被存放在这个链表里
**/
struct list_head rdllist;
/* RB tree root used to store monitored fd structs */
/**
所有待监听的epitem被存放在这个红黑树里
**/
struct rb_root rbr;
/*
* This is a single linked list that chains all the "struct epitem" that
* happened while transferring ready events to userspace w/out
* holding ->lock.
*/
/**
当event转移到用户空间时，这个单链表存放着所有struct epitem
**/
struct epitem *ovflist;
/* wakeup_source used when ep_scan_ready_list is running */
struct wakeup_source *ws; // TODO
/* The user that created the eventpoll descriptor */
/**
这里存放了一些用户变量，比如fd监听数量的最大值等
**/
struct user_struct *user;
struct file *file;
/* used to optimize loop detection check */ // TODO
int visited;
struct list_head visited_list_link;
};
/*
* Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree.
* Avoid increasing the size of this struct, there can be many thousands
* of these on a server and we do not want this to take another cache line.
*/
// epitem表示一个被监听的fd
struct epitem {
union {
/* RB tree node links this structure to the eventpoll RB tree */
/**
红黑树结点，当使用epoll_ctl()将一批fd加入到某个epollfd时，内核会分配一批epitem与fd一一对应，
并且以红黑树的形式来组织它们，tree的root被存放在struct eventpoll中．
**/
struct rb_node rbn;
/* Used to free the struct epitem */
struct rcu_head rcu; // TODO
};
/* List header used to link this structure to the eventpoll ready list */
/**
链表结点，所有已经ready的epitem都会被存放在eventpoll的rdllist链表中．
**/
struct list_head rdllink;
/*
* Works together "struct eventpoll"->ovflist in keeping the
* single linked chain of items.
*/
struct epitem *next; // 用于eventpoll的ovflist
/* The file descriptor information this item refers to */
/**
epitem对应的fd和struct file
**/
struct epoll_filefd ffd;
/* Number of active wait queue attached to poll operations */
int nwait; // 当前epitem被加入到多少个等待队列中
/* List containing poll wait queues */
struct list_head pwqlist;
/* The "container" of this item */
/**
当前epitem属于那个eventpoll
**/
struct eventpoll *ep;
/* List header used to link this item to the "struct file" items list */
struct list_head fllink;
/* wakeup_source used when EPOLLWAKEUP is set */
struct wakeup_source __rcu *ws;
/* The structure that describe the interested events and the source fd */
/**
当前epitem关心哪些event，这个数据是由执行epoll_ctl时从用户空间传递过来的
**/
struct epoll_event event;
};
struct epoll_filefd {
struct file *file;
int fd;
} __packed;
/* 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" */
struct epitem *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;
};
/* Used by the ep_send_events() function as callback private data */
struct ep_send_events_data {
int maxevents;
struct epoll_event __user *events;
};
/**
调用epoll_create()的实质，就是调用epoll_create1()．
**/
SYSCALL_DEFINE1(epoll_create, int, size)
{
if (size <= 0)
return -EINVAL;
return sys_epoll_create1(0);
}
/*
* Open an eventpoll file descriptor.
*/
SYSCALL_DEFINE1(epoll_create1, int, flags)
{
int error, fd;
struct eventpoll *ep = NULL;
struct file *file;
/* Check the EPOLL_* constant for consistency. */
BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
/**
对于epoll来说，目前唯一有效的FLAG是CLOSEXEC
**/
if (flags & ~EPOLL_CLOEXEC)
return -EINVAL;
/*
* Create the internal data structure ("struct eventpoll").
*/
/**
分配一个struct eventpoll，ep_alloc()的具体分析在下面
**/
error = ep_alloc(&ep);
if (error < 0)
return error;
/*
* Creates all the items needed to setup an eventpoll file. That is,
* a file structure and a free file descriptor.
*/
fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC)); // TODO
if (fd < 0) {
error = fd;
goto out_free_ep;
}
/**
创建一个匿名fd．
epollfd本身并不存在一个真正的文件与之对应，所以内核需要创建一个＂虚拟＂的文件，并为之分配
真正的struct file结构，并且具有真正的fd．
**/
file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
O_RDWR | (flags & O_CLOEXEC));
if (IS_ERR(file)) {
error = PTR_ERR(file);
goto out_free_fd;
}
ep->file = file;
fd_install(fd, file);
return fd;
out_free_fd:
put_unused_fd(fd);
out_free_ep:
ep_free(ep);
return error;
}
/**
分配一个eventpoll结构
**/
static int ep_alloc(struct eventpoll **pep)
{
int error;
struct user_struct *user;
struct eventpoll *ep;
/**
获取当前用户的一些信息，比如最大监听fd数目
**/
user = get_current_user();
error = -ENOMEM;
ep = kzalloc(sizeof(*ep), GFP_KERNEL); // 话说分配eventpoll对象是使用slab还是用buddy呢？TODO
if (unlikely(!ep))
goto free_uid;
/**
初始化
**/
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;
ep->user = user;
*pep = ep;
return 0;
free_uid:
free_uid(user);
return error;
}
/*
* The following function implements the controller interface for
* the eventpoll file that enables the insertion/removal/change of
* file descriptors inside the interest set.
*/
/**
调用epool_ctl来添加要监听的fd．
参数说明：
epfd，即epollfd
op，操作，ADD，MOD，DEL
fd，需要监听的文件描述符
event，关心的events
**/
SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
struct epoll_event __user *, event)
{
int error;
int full_check = 0;
struct fd f, tf;
struct eventpoll *ep;
struct epitem *epi;
struct epoll_event epds;
struct eventpoll *tep = NULL;
error = -EFAULT;
/**
错误处理以及
将event从用户空间拷贝到内核空间．
**/
if (ep_op_has_event(op) &&
copy_from_user(&epds, event, sizeof(struct epoll_event)))
goto error_return;
error = -EBADF;
/**
获取epollfd的file结构，该结构在epoll_create1()中，由函数anon_inode_getfile()分配
**/
f = fdget(epfd);
if (!f.file)
goto error_return;
/* Get the "struct file *" for the target file */
/**
获取待监听的fd的file结构
**/
tf = fdget(fd);
if (!tf.file)
goto error_fput;
/* The target file descriptor must support poll */
error = -EPERM;
/**
待监听的文件一定要支持poll．
话说什么情况下文件不支持poll呢？TODO
**/
if (!tf.file->f_op->poll)
goto error_tgt_fput;
/* Check if EPOLLWAKEUP is allowed */
if (ep_op_has_event(op))
ep_take_care_of_epollwakeup(&epds);
/*
* 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;
/**
epollfd不能监听自己
**/
if (f.file == tf.file || !is_file_epoll(f.file))
goto error_tgt_fput;
/*
* At this point it is safe to assume that the "private_data" contains
* our own data structure.
*/
/**
获取eventpoll结构，来自于epoll_create1()的分配
**/
ep = f.file->private_data;
/*
* When we insert an epoll file descriptor, inside another epoll file
* descriptor, there is the change of creating closed loops, which are
* better be handled here, than in more critical paths. While we are
* checking for loops we also determine the list of files reachable
* and hang them on the tfile_check_list, so we can check that we
* haven't created too many possible wakeup paths.
*
* We do not need to take the global 'epumutex' on EPOLL_CTL_ADD when
* the epoll file descriptor is attaching directly to a wakeup source,
* unless the epoll file descriptor is nested. The purpose of taking the
* 'epmutex' on add is to prevent complex toplogies such as loops and
* deep wakeup paths from forming in parallel through multiple
* EPOLL_CTL_ADD operations.
*/
/**
以下操作可能会修改数据结构内容，锁
**/
// TODO
mutex_lock_nested(&ep->mtx, 0);
if (op == EPOLL_CTL_ADD) {
if (!list_empty(&f.file->f_ep_links) ||
is_file_epoll(tf.file)) {
full_check = 1;
mutex_unlock(&ep->mtx);
mutex_lock(&epmutex);
if (is_file_epoll(tf.file)) {
error = -ELOOP;
if (ep_loop_check(ep, tf.file) != 0) {
clear_tfile_check_list();
goto error_tgt_fput;
}
} else
list_add(&tf.file->f_tfile_llink,
&tfile_check_list);
mutex_lock_nested(&ep->mtx, 0);
if (is_file_epoll(tf.file)) {
tep = tf.file->private_data;
mutex_lock_nested(&tep->mtx, 1);
}
}
}
/*
* 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.
*/
/**
对于每一个监听的fd，内核都有分配一个epitem结构，并且不允许重复分配，所以要查找该fd是否
已经存在．
ep_find()即在红黑树中查找，时间复杂度为O(lgN)．
**/
epi = ep_find(ep, tf.file, fd);
error = -EINVAL;
switch (op) {
/**
首先关心添加
**/
case EPOLL_CTL_ADD:
if (!epi) {
/**
如果ep_find()没有找到相关的epitem，证明是第一次插入．
在此可以看到，内核总会关心POLLERR和POLLHUP．
**/
epds.events |= POLLERR | POLLHUP;
/**
红黑树插入，ep_insert()的具体分析在下面
**/
error = ep_insert(ep, &epds, tf.file, fd, full_check);
} else
/**
如果找到了，则是重复添加
**/
error = -EEXIST;
if (full_check) // TODO
clear_tfile_check_list();
break;
case EPOLL_CTL_DEL:
/**
删除
**/
if (epi)
error = ep_remove(ep, epi);
else
error = -ENOENT;
break;
case EPOLL_CTL_MOD:
/**
修改
**/
if (epi) {
epds.events |= POLLERR | POLLHUP;
error = ep_modify(ep, epi, &epds);
} else
error = -ENOENT;
break;
}
if (tep != NULL)
mutex_unlock(&tep->mtx);
mutex_unlock(&ep->mtx); // 解锁
error_tgt_fput:
if (full_check)
mutex_unlock(&epmutex);
fdput(tf);
error_fput:
fdput(f);
error_return:
return error;
}
/*
* Must be called with "mtx" held.
*/
/**
ep_insert()在epoll_ctl()中被调用，其工作是往epollfd的红黑树中添加一个待监听fd．
**/
static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
struct file *tfile, int fd, int full_check)
{
int error, revents, pwake = 0;
unsigned long flags;
long user_watches;
struct epitem *epi;
struct ep_pqueue epq;
/**
struct ep_pqueue的定义如下：
@ fs/eventpoll.c
// Wrapper struct used by poll queueing
struct ep_pqueue {
poll_table pt;
struct epitem *epi;
};
**/
/**
查看是否达到当前用户的最大监听数
**/
user_watches = atomic_long_read(&ep->user->epoll_watches);
if (unlikely(user_watches >= max_user_watches))
return -ENOSPC;
/**
从slab中分配一个epitem
**/
if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
return -ENOMEM;
/* Item initialization follow here ... */
/**
相关数据成员的初始化
**/
INIT_LIST_HEAD(&epi->rdllink);
INIT_LIST_HEAD(&epi->fllink);
INIT_LIST_HEAD(&epi->pwqlist);
epi->ep = ep;
/**
在该epitem中保存待监听的fd和它的file结构．
**/
ep_set_ffd(&epi->ffd, tfile, fd);
epi->event = *event;
epi->nwait = 0;
epi->next = EP_UNACTIVE_PTR;
if (epi->event.events & EPOLLWAKEUP) {
error = ep_create_wakeup_source(epi);
if (error)
goto error_create_wakeup_source;
} else {
RCU_INIT_POINTER(epi->ws, NULL);
}
/* Initialize the poll table using the queue callback */
epq.epi = epi;
/**
初始化一个poll_table，
其实质是指定调用poll_wait()时(不是epoll_wait)的回调函数，以及我们关心哪些event．
ep_ptable_queue_proc()就是我们的回调函数，初值是所有event都关心．
ep_ptable_queue_proc()的具体分析在下面．
**/
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.
*/
revents = ep_item_poll(epi, &epq.pt);
/**
ep_item_poll()的定义如下：
@ fs/eventpoll.c
static inline unsigned int ep_item_poll(struct epitem *epi, poll_table *pt)
{
pt->_key = epi->event.events;
return epi->ffd.file->f_op->poll(epi->ffd.file, pt) & epi->event.events;
}
**/
/**
f_op->poll()一般来说只是个wrapper，它会调用真正的poll实现．
拿UDP的socket来举例，调用流程如下：
f_op->poll()，sock_poll()，udp_poll()，datagram_poll()，sock_poll_wait()，
最后调用到上面指定的ep_ptable_queue_proc()．
完成这一步，该epitem就跟这个socket关联起来了，当后者有状态变化时，会通过ep_poll_callback()
来通知．
所以，f_op->poll()做了两件事情：
1．将该epitem和这个待监听的fd关联起来；
2．查询这个待监听的fd是否已经有event已经ready了，有的话就将event返回．
**/
/*
* 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.
*/
error = -ENOMEM;
if (epi->nwait < 0)
goto error_unregister;
/* Add the current item to the list of active epoll hook for this file */
/**
把每个文件和对应的epitem关联起来
**/
spin_lock(&tfile->f_lock);
list_add_tail_rcu(&epi->fllink, &tfile->f_ep_links);
spin_unlock(&tfile->f_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.
*/
/**
将epitem插入到eventpoll的红黑树中
**/
ep_rbtree_insert(ep, epi);
/* now check if we've created too many backpaths */
error = -EINVAL;
if (full_check && reverse_path_check())
goto error_remove_epi;
/* We have to drop the new item inside our item list to keep track of it */
spin_lock_irqsave(&ep->lock, flags); // TODO
/* If the file is already "ready" we drop it inside the ready list */
/**
在这里，如果待监听的fd已经有事件发生，就去处理一下
**/
if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
/**
将当前的epitem加入到ready list中去
**/
list_add_tail(&epi->rdllink, &ep->rdllist);
ep_pm_stay_awake(epi);
/* Notify waiting tasks that events are available */
/**
哪个进程在调用epoll_wait()，就唤醒它
**/
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
/**
先不通知对eventpoll进行poll的进程
**/
if (waitqueue_active(&ep->poll_wait))
pwake++;
}
spin_unlock_irqrestore(&ep->lock, flags);
atomic_long_inc(&ep->user->epoll_watches);
/* We have to call this outside the lock */
if (pwake)
/**
安全地通知对eventpoll进行poll的进程
**/
ep_poll_safewake(&ep->poll_wait);
return 0;
error_remove_epi:
spin_lock(&tfile->f_lock);
list_del_rcu(&epi->fllink);
spin_unlock(&tfile->f_lock);
rb_erase(&epi->rbn, &ep->rbr);
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);
wakeup_source_unregister(ep_wakeup_source(epi));
error_create_wakeup_source:
kmem_cache_free(epi_cache, epi);
return error;
}
/*
* This is the callback that is used to add our wait queue to the
* target file wakeup lists.
*/
/**
该函数在调用f_op->poll()时被调用．
其作用是当epoll主动poll某个待监听fd时，将epitem和该fd关联起来．
关联的方法是使用等待队列．
**/
static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
poll_table *pt)
{
struct epitem *epi = ep_item_from_epqueue(pt);
struct eppoll_entry *pwq;
/**
@ fs/eventpoll.c
// 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"
struct epitem *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;
};
**/
if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
/**
初始化等待队列，指定ep_poll_callback()为唤醒时的回调函数．
当监听的fd发生状态改变时，即队列头被唤醒时，指定的回调函数会被调用．
**/
init_waitqueue_func_entry(&pwq->wait, ep_poll_callback); // ep_poll_callback()的具体分析在下面
pwq->whead = whead;
pwq->base = epi;
add_wait_queue(whead, &pwq->wait);
list_add_tail(&pwq->llink, &epi->pwqlist);
epi->nwait++;
} else {
/* We have to signal that an error occurred */
epi->nwait = -1;
}
}
/*
* This is the callback that is passed to the wait queue wakeup
* mechanism. It is called by the stored file descriptors when they
* have events to report.
*/
/**
这是一个关键的回调函数．
当被监听的fd发生状态改变时，该函数会被调用．
参数key指向events．
**/
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); // 从等待队列获取epitem
struct eventpoll *ep = 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;
/*
* Check the events coming with the callback. At this stage, not
* every device reports the events in the "key" parameter of the
* callback. We need to be able to handle both cases here, hence the
* test for "key" != NULL before the event match test.
*/
/**
没有我们关心的event
**/
if (key && !((unsigned long) key & epi->event.events))
goto out_unlock;
/*
* If we are transferring 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 happen during that period of time are
* chained in ep->ovflist and requeued later on.
*/
/**
如果该函数被调用时，epoll_wait()已经返回了，
即此时应用程序已经在循环中获取events了，
这种情况下，内核将此刻发生状态改变的epitem用一个单独的链表保存起来，并且在下一次epoll_wait()
时返回给用户．这个单独的链表就是ovflist．
*/
if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
if (epi->next == EP_UNACTIVE_PTR) {
epi->next = ep->ovflist;
ep->ovflist = epi;
if (epi->ws) {
/*
* Activate ep->ws since epi->ws may get
* deactivated at any time.
*/
__pm_stay_awake(ep->ws);
}
}
goto out_unlock;
}
/* If this file is already in the ready list we exit soon */
/**
将当前epitem添加到ready list中
**/
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
ep_pm_stay_awake_rcu(epi);
}
/*
* Wake up ( if active ) both the eventpoll wait list and the ->poll()
* wait list.
*/
/**
唤醒调用epoll_wait()的进程
**/
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
/**
先不通知对eventpoll进行poll的进程
**/
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)
/**
安全地通知对eventpoll进行poll的进程
**/
ep_poll_safewake(&ep->poll_wait);
if ((unsigned long)key & POLLFREE) {
/*
* If we race with ep_remove_wait_queue() it can miss
* ->whead = NULL and do another remove_wait_queue() after
* us, so we can't use __remove_wait_queue().
*/
list_del_init(&wait->task_list);
/*
* ->whead != NULL protects us from the race with ep_free()
* or ep_remove(), ep_remove_wait_queue() takes whead->lock
* held by the caller. Once we nullify it, nothing protects
* ep/epi or even wait.
*/
smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL);
}
return 1;
}
/*
* Implement the event wait interface for the eventpoll file. It is the kernel
* part of the user space epoll_wait(2).
*/
SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,
int, maxevents, int, timeout)
{
int error;
struct fd f;
struct eventpoll *ep;
/* The maximum number of event must be greater than zero */
if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
return -EINVAL;
/* Verify that the area passed by the user is writeable */
/**
内核要验证这一段用户空间的内存是不是有效的，可写的．
**/
if (!access_ok(VERIFY_WRITE, events, maxevents * sizeof(struct epoll_event)))
return -EFAULT;
/* Get the "struct file *" for the eventpoll file */
/**
获取epollfd的file结构
**/
f = fdget(epfd);
if (!f.file)
return -EBADF;
/*
* We have to check that the file structure underneath the fd
* the user passed to us _is_ an eventpoll file.
*/
error = -EINVAL;
/**
检查它是不是一个真正的epollfd
**/
if (!is_file_epoll(f.file))
goto error_fput;
/*
* At this point it is safe to assume that the "private_data" contains
* our own data structure.
*/
/**
获取eventpoll结构
**/
ep = f.file->private_data;
/* Time to fish for events ... */
/**
睡眠，等待事件到来．
ep_poll()的具体分析在下面．
**/
error = ep_poll(ep, events, maxevents, timeout);
error_fput:
fdput(f);
return error;
}
/**
* ep_poll - Retrieves ready events, and delivers them to the caller supplied
* event buffer.
*
* @ep: Pointer to the eventpoll context.
* @events: Pointer to the userspace buffer where the ready events should be
* stored.
* @maxevents: Size (in terms of number of events) of the caller event buffer.
* @timeout: Maximum timeout for the ready events fetch operation, in
* milliseconds. If the @timeout is zero, the function will not block,
* while if the @timeout is less than zero, the function will block
* until at least one event has been retrieved (or an error
* occurred).
*
* Returns: Returns the number of ready events which have been fetched, or an
* error code, in case of error.
*/
/**
执行epoll_wait()的进程在该函数进入休眠状态．
**/
static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
int maxevents, long timeout)
{
int res = 0, eavail, timed_out = 0;
unsigned long flags;
long slack = 0;
wait_queue_t wait;
ktime_t expires, *to = NULL;
if (timeout > 0) {
/**
计算睡眠时间
**/
struct timespec end_time = ep_set_mstimeout(timeout);
slack = select_estimate_accuracy(&end_time);
to = &expires;
*to = timespec_to_ktime(end_time);
} else if (timeout == 0) {
/**
已经超时，直接检查ready list
**/
/*
* Avoid the unnecessary trip to the wait queue loop, if the
* caller specified a non blocking operation.
*/
timed_out = 1;
spin_lock_irqsave(&ep->lock, flags);
goto check_events;
}
fetch_events:
spin_lock_irqsave(&ep->lock, flags);
/**
没有可用的事件，即ready list和ovflist均为空．
**/
if (!ep_events_available(ep)) {
/*
* We don't have any available event to return to the caller.
* We need to sleep here, and we will be wake up by
* ep_poll_callback() when events will become available.
*/
/**
初始化一个等待队列成员，current是当前进程．
然后把该等待队列成员添加到ep的等待队列中，即当前进程把自己添加到等待队列中．
**/
init_waitqueue_entry(&wait, current);
__add_wait_queue_exclusive(&ep->wq, &wait);
for (;;) {
/*
* We don't want to sleep if the ep_poll_callback() sends us
* a wakeup in between. That's why we set the task state
* to TASK_INTERRUPTIBLE before doing the checks.
*/
/**
将当前进程的状态设置为睡眠时可以被信号唤醒．
仅仅是状态设置，还没有睡眠．
**/
set_current_state(TASK_INTERRUPTIBLE);
/**
如果此时，ready list已经有成员了，或者已经超时，则不进入睡眠．
**/
if (ep_events_available(ep) || timed_out)
break;
/**
如果有信号产生，不进入睡眠．
**/
if (signal_pending(current)) {
res = -EINTR;
break;
}
spin_unlock_irqrestore(&ep->lock, flags);
/**
挂起当前进程，等待被唤醒或超时
**/
if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
timed_out = 1;
spin_lock_irqsave(&ep->lock, flags);
}
__remove_wait_queue(&ep->wq, &wait); // 把当前进程从该epollfd的等待队列中删除．
__set_current_state(TASK_RUNNING); // 将当前进程的状态设置为可运行．
}
check_events:
/* Is it worth to try to dig for events ? */
eavail = ep_events_available(ep);
spin_unlock_irqrestore(&ep->lock, flags);
/*
* Try to transfer events to user space. In case we get 0 events and
* there's still timeout left over, we go trying again in search of
* more luck.
*/
/**
如果一切正常，并且有event发生，则拷贝数据给用户空间
**/
// ep_send_events()的具体分析在下面
if (!res && eavail &&
!(res = ep_send_events(ep, events, maxevents)) && !timed_out)
goto fetch_events;
return res;
}
static int ep_send_events(struct eventpoll *ep,
struct epoll_event __user *events, int maxevents)
{
struct ep_send_events_data esed;
/**
@ fs/eventpoll.c
// Used by the ep_send_events() function as callback private data
struct ep_send_events_data {
int maxevents;
struct epoll_event __user *events;
};
**/
esed.maxevents = maxevents;
esed.events = events;
// ep_scan_ready_list()的具体分析在下面
return ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0, false);
}
/**
* ep_scan_ready_list - Scans the ready list in a way that makes possible for
* the scan code, to call f_op->poll(). Also allows for
* O(NumReady) performance.
*
* @ep: Pointer to the epoll private data structure.
* @sproc: Pointer to the scan callback.
* @priv: Private opaque data passed to the @sproc callback.
* @depth: The current depth of recursive f_op->poll calls.
* @ep_locked: caller already holds ep->mtx
*
* Returns: The same integer error code returned by the @sproc callback.
*/
static int ep_scan_ready_list(struct eventpoll *ep,
int (*sproc)(struct eventpoll *,
struct list_head *, void *),
void *priv, int depth, bool ep_locked)
{
int error, pwake = 0;
unsigned long flags;
struct epitem *epi, *nepi;
LIST_HEAD(txlist);
/*
* We need to lock this because we could be hit by
* eventpoll_release_file() and epoll_ctl().
*/
if (!ep_locked)
mutex_lock_nested(&ep->mtx, depth);
/*
* Steal the ready list, and re-init the original one to the
* empty list. Also, set ep->ovflist to NULL so that events
* happening while looping w/out locks, are not lost. We cannot
* have the poll callback to queue directly on ep->rdllist,
* because we want the "sproc" callback to be able to do it
* in a lockless way.
*/
spin_lock_irqsave(&ep->lock, flags);
/**
将ready list上的epitem(即监听事件发生状态改变的epitem)移动到txlist，
并且将ready list清空．
**/
list_splice_init(&ep->rdllist, &txlist);
/**
改变ovflist的值．
在上面的ep_poll_callback()中可以看到，如果ovflist != EP_UNACTIVE_PTR，当等待队列成员被激活时，
就会将对应的epitem加入到ep->ovflist中，否则加入到ep->rdllist中．
所以这里是为了防止把新来的发生状态改变的epitem加入到ready list中．
**/
ep->ovflist = NULL;
spin_unlock_irqrestore(&ep->lock, flags);
/*
* Now call the callback function.
*/
/**
调用扫描函数处理txlist．
该扫描函数就是ep_send_events_proc．具体分析在下面．
**/
error = (*sproc)(ep, &txlist, priv);
spin_lock_irqsave(&ep->lock, flags);
/*
* During the time we spent inside the "sproc" callback, some
* other events might have been queued by the poll callback.
* We re-insert them inside the main ready-list here.
*/
/**
在调用sproc()期间，可能会有新的事件发生(被添加到ovflist中)，遍历这些发生新事件的epitem，
将它们插入到ready list中．
**/
for (nepi = ep->ovflist; (epi = nepi) != NULL;
nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
/**
@ fs/eventpoll.c
#define EP_UNACTIVE_PTR ((void *) -1L)
**/
/*
* We need to check if the item is already in the list.
* During the "sproc" callback execution time, items are
* queued into ->ovflist but the "txlist" might already
* contain them, and the list_splice() below takes care of them.
*/
/**
epitem不在ready list？插入！
**/
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
ep_pm_stay_awake(epi);
}
}
/*
* We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
* releasing the lock, events will be queued in the normal way inside
* ep->rdllist.
*/
/**
还原ovflist的状态
**/
ep->ovflist = EP_UNACTIVE_PTR;
/*
* Quickly re-inject items left on "txlist".
*/
/**
将上次没有处理完的epitem，重新插入到ready list中．
**/
list_splice(&txlist, &ep->rdllist);
__pm_relax(ep->ws);
/**
如果ready list不为空，唤醒．
**/
if (!list_empty(&ep->rdllist)) {
/*
* Wake up (if active) both the eventpoll wait list and
* the ->poll() wait list (delayed after we release the lock).
*/
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
if (waitqueue_active(&ep->poll_wait))
pwake++;
}
spin_unlock_irqrestore(&ep->lock, flags);
if (!ep_locked)
mutex_unlock(&ep->mtx);
/* We have to call this outside the lock */
if (pwake)
ep_poll_safewake(&ep->poll_wait);
return error;
}
/**
该函数作为callback在ep_scan_ready_list()中被调用．
head是一个链表头，链接着已经ready了的epitem．
这个链表不是eventpoll的ready list，而是上面函数中的txlist．
**/
static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
void *priv)
{
struct ep_send_events_data *esed = priv;
int eventcnt;
unsigned int revents;
struct epitem *epi;
struct epoll_event __user *uevent;
struct wakeup_source *ws;
poll_table pt;
init_poll_funcptr(&pt, NULL);
/*
* We can loop without lock because we are passed a task private list.
* Items cannot vanish during the loop because ep_scan_ready_list() is
* holding "mtx" during this call.
*/
/**
遍历整个链表
**/
for (eventcnt = 0, uevent = esed->events;
!list_empty(head) && eventcnt < esed->maxevents;) {
/**
取出第一个结点
**/
epi = list_first_entry(head, struct epitem, rdllink);
/*
* Activate ep->ws before deactivating epi->ws to prevent
* triggering auto-suspend here (in case we reactive epi->ws
* below).
*
* This could be rearranged to delay the deactivation of epi->ws
* instead, but then epi->ws would temporarily be out of sync
* with ep_is_linked().
*/
// TODO
ws = ep_wakeup_source(epi);
if (ws) {
if (ws->active)
__pm_stay_awake(ep->ws);
__pm_relax(ws);
}
/**
从ready list中删除该结点
**/
list_del_init(&epi->rdllink);
/**
获取ready事件掩码
**/
revents = ep_item_poll(epi, &pt);
/**
ep_item_poll()的具体分析在上面的ep_insert()中．
**/
/*
* If the event mask intersect the caller-requested one,
* deliver the event to userspace. Again, ep_scan_ready_list()
* is holding "mtx", so no operations coming from userspace
* can change the item.
*/
if (revents) {
/**
将ready事件和用户传入的数据都拷贝到用户空间
**/
if (__put_user(revents, &uevent->events) ||
__put_user(epi->event.data, &uevent->data)) {
list_add(&epi->rdllink, head);
ep_pm_stay_awake(epi);
return eventcnt ? eventcnt : -EFAULT;
}
eventcnt++;
uevent++;
if (epi->event.events & EPOLLONESHOT)
epi->event.events &= EP_PRIVATE_BITS;
else if (!(epi->event.events & EPOLLET)) {
/**
边缘触发(ET)和水平触发(LT)的区别：
如果是ET，就绪epitem不会再次被加入到ready list中，除非fd再次发生状态改变，ep_poll_callback被调用．
如果是LT，不论是否还有有效的事件和数据，epitem都会被再次加入到ready list中，在下次epoll_wait()时会
立即返回，并通知用户空间．当然如果这个被监听的fd确实没有事件和数据，epoll_wait()会返回一个0．
**/
/*
* If this file has been added with Level
* Trigger mode, we need to insert back inside
* the ready list, so that the next call to
* epoll_wait() will check again the events
* availability. At this point, no one can insert
* into ep->rdllist besides us. The epoll_ctl()
* callers are locked out by
* ep_scan_ready_list() holding "mtx" and the
* poll callback will queue them in ep->ovflist.
*/
list_add_tail(&epi->rdllink, &ep->rdllist);
ep_pm_stay_awake(epi);
}
}
}
return eventcnt;
}
/**
该函数在epollfd被close时调用，其工作是释放一些资源．
**/
static void ep_free(struct eventpoll *ep)
{
struct rb_node *rbp;
struct epitem *epi;
/* We need to release all tasks waiting for these file */
if (waitqueue_active(&ep->poll_wait))
ep_poll_safewake(&ep->poll_wait);
/*
* We need to lock this because we could be hit by
* eventpoll_release_file() while we're freeing the "struct eventpoll".
* We do not need to hold "ep->mtx" here because the epoll file
* is on the way to be removed and no one has references to it
* anymore. The only hit might come from eventpoll_release_file() but
* holding "epmutex" is sufficient here.
*/
mutex_lock(&epmutex);
/*
* Walks through the whole tree by unregistering poll callbacks.
*/
for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
epi = rb_entry(rbp, struct epitem, rbn);
ep_unregister_pollwait(ep, epi);
cond_resched();
}
/*
* Walks through the whole tree by freeing each "struct epitem". At this
* point we are sure no poll callbacks will be lingering around, and also by
* holding "epmutex" we can be sure that no file cleanup code will hit
* us during this operation. So we can avoid the lock on "ep->lock".
* We do not need to lock ep->mtx, either, we only do it to prevent
* a lockdep warning.
*/
mutex_lock(&ep->mtx);
/**
在epoll_ctl()中被添加的监听fd，在这里被关闭．
**/
while ((rbp = rb_first(&ep->rbr)) != NULL) {
epi = rb_entry(rbp, struct epitem, rbn);
ep_remove(ep, epi);
cond_resched();
}
mutex_unlock(&ep->mtx);
mutex_unlock(&epmutex);
mutex_destroy(&ep->mtx);
free_uid(ep->user);
wakeup_source_unregister(ep->ws);
kfree(ep);
}2.实现过程
一. epoll_create
- 调用ep_alloc()来创建一个struct eventpoll对象．ep_alloc()的执行过程如下：
1a. 获取当前用户的一些信息．
1b. 分配一个struct eventpoll对象．
1c. 初始化相关数据成员，如等待队列，就绪链表，红黑树． - 创建一个匿名fd和与之对应的struct file对象．
- 将该eventpoll和该file关联起来，eventpoll对象保存在file对象的private_data指针中．
二. epoll_ctl
- 将event拷贝到内核空间．
- 判断加入的fd是否支持poll操作．
- 根据用户传入的op参数，以及是否在eventpoll的红黑树中找到该fd的结点，来执行相应的操作(插入，删除，修改)．拿插入举例，执行ep_insert()：
3a. 在slab缓存中分配一个epitem对象，并初始化相关数据成员，如保存待监听的fd和它的file结构．
3b. 指定调用poll_wait()时(再次强调，不是epoll_wait)时的回调函数，用于数据就绪时唤醒进程．(其实质是初始化文件的等待队列，将进程加入到等待队列)．
3c. 到此该epitem就和这个待监听的fd关联起来了．
3d. 将该epitem插入到eventpoll的红黑树中．
三. epoll_wait
- 调用ep_poll()：
1a. 计算睡眠时间(如果有)．
1b. 判断eventpoll的就绪链表是否为空，不为空则直接处理而不是睡眠．
1c. 将当前进程添加到eventpoll的等待队列中．
1d. 进入循环．
1e. 将当前进程设置成TASK_INTERRUPTIBLE状态，然后判断是否有信号到来，如果没有，则进入睡眠．
1f. 如果超时或被信号唤醒，则跳出循环．
1g. 将当前进程从等待队列中删除，并把其状态设置成TASK_RUNNING．
1h. 将数据拷贝给用户空间．拷贝的过程是先把ready list转移到中间链表，然后遍历中间链表拷贝到用户空间，并且判断每个结点是否水平触发，是则再次插入
到ready list．
3.小结
从上述分析可见，相对于select/poll，epoll的高效体现在：
- fd只拷贝一次，即调用epoll_ctl()时把fd拷贝到内核空间．
- epoll轮询的是就绪链表，而不是所有fd．
最后
本以为两天时间就可以分析完这些源码并完成这篇博文，谁知道花了整整一个星期．不过收获匪浅．
在此留下两个问题：
- 因为目前我的知识面有限，上述源码留下一些TODO，日后会一一完善．
- 话说epoll有什么缺点呢？
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