struct pollfd {
int fd; //当前描述符
short events; //进程关心的该描述符的事件
short revents; //返回的事件
};
asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,
long timeout_msecs)
{
s64 timeout_jiffies;
//超时时间处理
if (timeout_msecs > 0) {
#if HZ > 1000
/* We can only overflow if HZ > 1000 */
if (timeout_msecs / 1000 > (s64)0x7fffffffffffffffULL / (s64)HZ)
timeout_jiffies = -1;
else
#endif
timeout_jiffies = msecs_to_jiffies(timeout_msecs);
} else {
/* Infinite (< 0) or no (0) timeout */
timeout_jiffies = timeout_msecs;
}
//实际处理函数
return do_sys_poll(ufds, nfds, &timeout_jiffies);
}
struct poll_list {
struct poll_list *next;
int len;
struct pollfd entries[0];
};
int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds, s64 *timeout)
{
struct poll_wqueues table;
int fdcount, err;
unsigned int i;
struct poll_list *head;
struct poll_list *walk;
/* Allocate small arguments on the stack to save memory and be
faster - use long to make sure the buffer is aligned properly
on 64 bit archs to avoid unaligned access */
long stack_pps[POLL_STACK_ALLOC/sizeof(long)]; //栈的分配会更快
struct poll_list *stack_pp = NULL;
//检查描述符个数是否超过系统的限制
/* Do a sanity check on nfds ... */
if (nfds > current->signal->rlim[RLIMIT_NOFILE].rlim_cur)
return -EINVAL;
//首先是一个初始化工作, 主要初始化poll_table这个函数指针
poll_initwait(&table);
head = NULL;
walk = NULL;
i = nfds;
err = -ENOMEM;
//这个循环所作的工作就是将从用户传过来的多个pollfd结构信息拷贝到内核，
//由于可能结构的个数可能超过一页内存所能存储的范围，所以就用了循环来完成，
//每次拷贝一页内存能装载的个数。并且再将它们用链表链起来。
while(i!=0) {
struct poll_list *pp;
int num, size;
if (stack_pp == NULL)
num = N_STACK_PPS;
else
num = POLLFD_PER_PAGE; //这里保证kmalloc分配的空间不会超过一个页面
if (num > i)
num = i;
size = sizeof(struct poll_list) + sizeof(struct pollfd)*num;
//如果描述符的个数比较小时,或在比较大的时候,第一次会使用栈来存储
if (!stack_pp)
stack_pp = pp = (struct poll_list *)stack_pps;
else {
pp = kmalloc(size, GFP_KERNEL);
if (!pp)
goto out_fds;
}
pp->next=NULL;
pp->len = num;
if (head == NULL)
head = pp;
else
walk->next = pp;
walk = pp;
if (copy_from_user(pp->entries, ufds + nfds-i,
sizeof(struct pollfd)*num)) {
err = -EFAULT;
goto out_fds;
}
i -= pp->len;
}
//真正的POLL操作，返回的结果在head中
fdcount = do_poll(nfds, head, &table, timeout);
//双重循环，将事件拷贝回给用户空间
/* OK, now copy the revents fields back to user space. */
walk = head;
err = -EFAULT;
while(walk != NULL) {
struct pollfd *fds = walk->entries;
int j;
for (j=0; j < walk->len; j++, ufds++) {
if(__put_user(fds[j].revents, &ufds->revents))
goto out_fds;
}
walk = walk->next;
}
err = fdcount;
if (!fdcount && signal_pending(current))
err = -EINTR;
//以下是释放空间
out_fds:
walk = head;
while(walk!=NULL) {
struct poll_list *pp = walk->next;
if (walk != stack_pp)
kfree(walk);
walk = pp;
}
poll_freewait(&table);
return err;
}
//这个函数就是将当前进程加入等待队列，这个等待队列由驱动或文件系统或网络协议栈来提供
//这个函数是由驱动的file->poll中调用poll_wait()来间接调用的。
/* Add a new entry */
static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
poll_table *p)
{
struct poll_table_entry *entry = poll_get_entry(p);
if (!entry)
return;
get_file(filp);
entry->filp = filp;
entry->wait_address = wait_address;
init_waitqueue_entry(&entry->wait, current);
add_wait_queue(wait_address,&entry->wait);
}
void poll_initwait(struct poll_wqueues *pwq)
{
//在poll()中初始化为__pollwait(),注意在epoll中又会不同
init_poll_funcptr(&pwq->pt, __pollwait);
pwq->error = 0;
pwq->table = NULL;
pwq->inline_index = 0;
}
===========================================
static int do_poll(unsigned int nfds, struct poll_list *list,
struct poll_wqueues *wait, s64 *timeout)
{
int count = 0;
poll_table* pt = &wait->pt;
/* Optimise the no-wait case */
if (!(*timeout)) //进程不设超时
pt = NULL;
for (;;) {
struct poll_list *walk;
long __timeout;
也是一个双重循环，处理每个文件描述符事件
set_current_state(TASK_INTERRUPTIBLE);
for (walk = list; walk != NULL; walk = walk->next) {
struct pollfd * pfd, * pfd_end;
pfd = walk->entries;
pfd_end = pfd + walk->len;
for (; pfd != pfd_end; pfd++) {
/*
* Fish for events. If we found one, record it
* and kill the poll_table, so we don't
* needlessly register any other waiters after
* this. They'll get immediately deregistered
* when we break out and return.
*/
if (do_pollfd(pfd, pt)) { //处理每个文件描述符
count++;
pt = NULL;
}
}
}
//超时处理
/*
* All waiters have already been registered, so don't provide
* a poll_table to them on the next loop iteration.
*/
pt = NULL;
if (count || !*timeout || signal_pending(current))
break;
count = wait->error;
if (count)
break;
if (*timeout < 0) {
/* Wait indefinitely */
__timeout = MAX_SCHEDULE_TIMEOUT;
} else if (unlikely(*timeout >= (s64)MAX_SCHEDULE_TIMEOUT-1)) {
/*
* Wait for longer than MAX_SCHEDULE_TIMEOUT. Do it in
* a loop
*/
__timeout = MAX_SCHEDULE_TIMEOUT - 1;
*timeout -= __timeout;
} else {
__timeout = *timeout;
*timeout = 0;
}
//进程切换
__timeout = schedule_timeout(__timeout);
//进程被唤醒, 继续执行
if (*timeout >= 0)
*timeout += __timeout;
}
__set_current_state(TASK_RUNNING);
return count;
}
/*
* Fish for pollable events on the pollfd->fd file descriptor. We're only
* interested in events matching the pollfd->events mask, and the result
* matching that mask is both recorded in pollfd->revents and returned. The
* pwait poll_table will be used by the fd-provided poll handler for waiting,
* if non-NULL.
*/
static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
{
unsigned int mask;
int fd;
mask = 0;
fd = pollfd->fd;
if (fd >= 0) {
int fput_needed;
struct file * file;
file = fget_light(fd, &fput_needed);
mask = POLLNVAL;
if (file != NULL) {
mask = DEFAULT_POLLMASK;
//调用驱动或文件系统的poll函数, 是否将当前进程加入驱动的等待队列,
//取决是file->poll()第二个参数是否为空.
if (file->f_op && file->f_op->poll)
mask = file->f_op->poll(file, pwait);
/* Mask out unneeded events. */
mask &= pollfd->events | POLLERR | POLLHUP;
fput_light(file, fput_needed);
}
}
pollfd->revents = mask; //更新参数返回值
return mask; //如果可读/写返回非0值
}
=================================
驱动或文件系统的poll()实现原型:
test_poll(struct file *filep, poll_table *wait)
{
...
poll_wait(filep, &dev->wait_queue_head, wait);
...
if (dev->readable)
mask |= POLLIN | POLLRDNORM;
if (dev->writable)
mask |= POLLOUT | POLLWRNORM;
...
}
static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
if (p && wait_address)
p->qproc(filp, wait_address, p); //这个函数就是上面又poll_initwait()初始化的__pollwait()了.
}
来自：http://dev.firnow.com/course/6_system/linux/Linuxjs/2008929/146653.html
