理解select，poll，epoll实现分析

int select(int nfds,
fd_set *restrict readfds,
fd_set *restrict writefds,
fd_set *restrict errorfds,
struct timeval *restrict timeout);


SYSCALL_DEFINE5(select, int, n,
fd_set __user *, inp,
fd_set __user *, outp,
fd_set __user *, exp,
struct timeval __user *, tvp)
{
// …
ret = core_sys_select(n, inp, outp, exp, to);
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;
// …
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;
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);
// …
}


int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
{
// …
for (;;) {
// …
for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
// …
struct fd f;
f = fdget(i);
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,
bit, busy_flag);
// 对每个fd进行I/O事件检测
mask = (*f_op->poll)(f.file, wait);
}
fdput(f);
// …
}
}
// 退出循环体
if (retval || timed_out || signal_pending(current))
break;
// 进入休眠
if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,
to, slack))
timed_out = 1;
}
}


int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
ktime_t *expires, unsigned long slack)
{
int rc = -EINTR;// 休眠
set_current_state(state);
if (!pwq->triggered)
rc = schedule_hrtimeout_range(expires, slack, HRTIMER_MODE_ABS);
__set_current_state(TASK_RUNNING);/*
* Prepare for the next iteration.
*
* The following set_mb() serves two purposes. First, it's
* the counterpart rmb of the wmb in pollwake() such that data
* written before wake up is always visible after wake up.
* Second, the full barrier guarantees that triggered clearing
* doesn't pass event check of the next iteration. Note that
* this problem doesn't exist for the first iteration as
* add_wait_queue() has full barrier semantics.
*/
set_mb(pwq->triggered, 0);return rc;
}
EXPORT_SYMBOL(poll_schedule_timeout);


mask = (*f_op->poll)(f.file, wait);


struct file {
struct pathf_path;
struct inode*f_inode;/* cached value */
const struct file_operations*f_op;// …} __attribute__((aligned(4)));/* lest something weird decides that 2 is OK */


struct file_operations {
struct module *owner;
loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
int (*iterate) (struct file *, struct dir_context *);
// select()轮询设备fd的操作函数
unsigned int (*poll) (struct file *, struct poll_table_struct *);
// …
};

unsigned int (*poll) (struct file *filp, struct poll_table_struct *pwait);


struct scull_pipe {
wait_queue_head_t inq, outq; /* read and write queues */
char *buffer, *end; /* begin of buf, end of buf */
int buffersize; /* used in pointer arithmetic */
char *rp, *wp; /* where to read, where to write */
int nreaders, nwriters; /* number of openings for r/w */
struct fasync_struct *async_queue; /* asynchronous readers */
struct mutex mutex; /* mutual exclusion semaphore */
struct cdev cdev; /* Char device structure */
};


static unsigned int scull_p_poll(struct file *filp, poll_table *wait)
{
struct scull_pipe *dev = filp->private_data;
unsigned int mask = 0;/*
* The buffer is circular; it is considered full
* if "wp" is right behind "rp" and empty if the
* two are equal.
*/
mutex_lock(&dev->mutex);
poll_wait(filp, &dev->inq, wait);
poll_wait(filp, &dev->outq, wait);
if (dev->rp != dev->wp)
mask |= POLLIN | POLLRDNORM;/* readable */
if (spacefree(dev))
mask |= POLLOUT | POLLWRNORM;/* writable */
mutex_unlock(&dev->mutex);
return mask;
}


static ssize_t scull_p_write(struct file *filp, const char __user *buf, size_t count,
loff_t *f_pos)
{
// …
/* Make sure there's space to write */
// …
/* ok, space is there, accept something */
// …
/* finally, awake any reader */
wake_up_interruptible(&dev->inq); /* blocked in read() and select() */
// …
}


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);
}/*
* Do not touch the structure directly, use the access functions
* poll_does_not_wait() and poll_requested_events() instead.
*/
typedef struct poll_table_struct {
poll_queue_proc _qproc;
unsigned long _key;
} poll_table;/*
* structures and helpers for f_op->poll implementations
*/
typedef void (*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_table_struct *);


int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
{
struct poll_wqueues table;
poll_table *wait;
poll_initwait(&table);
wait = &table.pt;
// …
}void poll_initwait(struct poll_wqueues *pwq)
{
// 初始化poll_table里的函数指针
init_poll_funcptr(&pwq->pt, __pollwait);
pwq->polling_task = current;
pwq->triggered = 0;
pwq->error = 0;
pwq->table = NULL;
pwq->inline_index = 0;
}
EXPORT_SYMBOL(poll_initwait);static inline void init_poll_funcptr(poll_table *pt, poll_queue_proc qproc)
{
pt->_qproc = qproc;
pt->_key = ~0UL; /* all events enabled */
}


/* Add a new entry */
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;
// 把当前进程装到设备的等待队列
add_wait_queue(wait_address, &entry->wait);
}void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;wait->flags &= ~WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&q->lock, flags);
__add_wait_queue(q, wait);
spin_unlock_irqrestore(&q->lock, flags);
}
EXPORT_SYMBOL(add_wait_queue);static inline void __add_wait_queue(wait_queue_head_t *head, wait_queue_t *new)
{
list_add(&new->task_list, &head->task_list);
}/**
* Insert a new element after the given list head. The new element does not
* need to be initialised as empty list.
* The list changes from:
* head → some element → ...
* to
* head → new element → older element → ...
*
* Example:
* struct foo *newfoo = malloc(...);
* list_add(&newfoo->entry, &bar->list_of_foos);
*
* @param entry The new element to prepend to the list.
* @param head The existing list.
*/
static inline void
list_add(struct list_head *entry, struct list_head *head)
{
__list_add(entry, head, head->next);
}

#define __FD_SETSIZE1024typedef struct {
unsigned long fds_bits[__FD_SETSIZE / (8 * sizeof(long))];
} __kernel_fd_set;typedef __kernel_fd_setfd_set;


struct fd {
struct file *file;
unsigned int flags;
};


/*
* Structures and helpers for select/poll syscall
*/
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];
};struct poll_table_entry {
struct file *filp;
unsigned long key;
wait_queue_t wait;
wait_queue_head_t *wait_address;
};


struct __wait_queue_head {
spinlock_tlock;
struct list_headtask_list;
};
typedef struct __wait_queue_head wait_queue_head_t;