// @ 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NULLif (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.htypedef 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.hstruct fdtable {unsigned int max_fds;struct file __rcu **fd;      // current fd arrayunsigned 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.hstatic inlineint 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.hstruct 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.htypedef 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; // TODOunsigned long busy_end = 0;rcu_read_lock(); // TODOretval = 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.cvoid 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.hstatic 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errorretval++;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, rexpif (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()) { // TODOif (!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;
}

// @ 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 */ // TODOint 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)); // TODOif (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？TODOif (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epollfdop，操作，ADD，MOD，DELfd，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**/// TODOmutex_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;}} elselist_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) // TODOclear_tfile_check_list();break;case EPOLL_CTL_DEL:/**删除**/if (epi)error = ep_remove(ep, epi);elseerror = -ENOENT;break;case EPOLL_CTL_MOD:/**修改**/if (epi) {epds.events |= POLLERR | POLLHUP;error = ep_modify(ep, epi, &epds);} elseerror = -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 queueingstruct 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.cstatic 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 hooksstruct 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 itemwait_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epitemstruct 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 datastruct 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().*/// TODOws = 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);
}