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我们在Linux的应用编程中，经常会使用的进程间通讯，pipe是一种常见的匿名管道通讯， 具体Linux内核如何实现系统调用呢？下面我们一起来分析 fs/pipe.c中我们可以看到，pipe被定义为系统调用 SYSCALL_DEFINE2(pipe2, int __user *, fildes, int, flags)
{
struct file *files[2];
int fd[2];
int error;
error = __do_pipe_flags(fd, files, flags);
if (!error) {
if (unlikely(copy_to_user(fildes, fd, sizeof(fd)))) {
fput(files[0]);
fput(files[1]);
put_unused_fd(fd[0]);
put_unused_fd(fd[1]);
error = -EFAULT;
} else {
fd_install(fd[0], files[0]);
fd_install(fd[1], files[1]);
}
}
return error;
}
SYSCALL_DEFINE1(pipe, int __user *, fildes)
{
return sys_pipe2(fildes, 0);
}可以看到SYSCALL_DEFINE1(pipe, int __user *, fildes)定义了系统调用pipe，主要参数是用户空间整形指针，用于回传文件描述符。 static int __do_pipe_flags(int *fd, struct file **files, int flags)
{
int error;
int fdw, fdr;
if (flags & ~(O_CLOEXEC | O_NONBLOCK | O_DIRECT))
return -EINVAL;
error = create_pipe_files(files, flags);
if (error)
return error;
error = get_unused_fd_flags(flags);
if (error < 0)
goto err_read_pipe;
fdr = error;
error = get_unused_fd_flags(flags);
if (error < 0)
goto err_fdr;
fdw = error;
audit_fd_pair(fdr, fdw);
fd[0] = fdr;
fd[1] = fdw;
return 0;
err_fdr:
put_unused_fd(fdr);
err_read_pipe:
fput(files[0]);
fput(files[1]);
return error;
}从create_pipe_files(files, flags);可以看出，pipe的内核底层原理实质上是利用Linux文件子系统来实现管道的通讯，与实际文件不同的是没有向用户空间提供文件节点操作接口，在内存中直接简历文件，文件描述符等。 int create_pipe_files(struct file **res, int flags)
{
int err;
struct inode *inode = get_pipe_inode();
struct file *f;
struct path path;
if (!inode)
return -ENFILE;
err = -ENOMEM;
path.dentry = d_alloc_pseudo(pipe_mnt->mnt_sb, &empty_name);
if (!path.dentry)
goto err_inode;
path.mnt = mntget(pipe_mnt);
d_instantiate(path.dentry, inode);
f = alloc_file(&path, FMODE_WRITE, &pipefifo_fops);
if (IS_ERR(f)) {
err = PTR_ERR(f);
goto err_dentry;
}
f->f_flags = O_WRONLY | (flags & (O_NONBLOCK | O_DIRECT));
f->private_data = inode->i_pipe;
res[0] = alloc_file(&path, FMODE_READ, &pipefifo_fops);
if (IS_ERR(res[0])) {
err = PTR_ERR(res[0]);
goto err_file;
}
path_get(&path);
res[0]->private_data = inode->i_pipe;
res[0]->f_flags = O_RDONLY | (flags & O_NONBLOCK);
res[1] = f;
return 0;
err_file:
put_filp(f);
err_dentry:
free_pipe_info(inode->i_pipe);
path_put(&path);
return err;
err_inode:
free_pipe_info(inode->i_pipe);
iput(inode);
return err;
}通过 alloc_file(&path, FMODE_WRITE, &pipefifo_fops);可以看到，申请了内存file，通过第二个参数 FMODE_WRITE， FMODE_READ控制，该文件是读端还是写端，这就是我们为什么在编程过程中为什么要一端读一端写的原因，参数pipefifo_fops，这个函数回调集合很重要，它决定了上层读写最终调到内核的函数。 首先看一下 file申请过程，flags过滤 struct file *alloc_file(const struct path *path, fmode_t mode,
const struct file_operations *fop)
{
struct file *file;
file = get_empty_filp();
if (IS_ERR(file))
return file;
file->f_path = *path;
file->f_inode = path->dentry->d_inode;
file->f_mapping = path->dentry->d_inode->i_mapping;
file->f_wb_err = filemap_sample_wb_err(file->f_mapping);
if ((mode & FMODE_READ) &&
likely(fop->read || fop->read_iter))
mode |= FMODE_CAN_READ;
if ((mode & FMODE_WRITE) &&
likely(fop->write || fop->write_iter))
mode |= FMODE_CAN_WRITE;
file->f_mode = mode;
file->f_op = fop;
if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
i_readcount_inc(path->dentry->d_inode);
return file;
}
EXPORT_SYMBOL(alloc_file);可以看到，通过不同标志位，设置不同文件读写模式，并将f_op指向pipefifo_fops结构体。 const struct file_operations pipefifo_fops = {
.open = fifo_open,
.llseek = no_llseek,
.read_iter = pipe_read,
.write_iter = pipe_write,
.poll = pipe_poll,
.unlocked_ioctl = pipe_ioctl,
.release = pipe_release,
.fasync = pipe_fasync,
};最终整个管道的读写都会调到pipe_read、pipe_write回调函数中 static ssize_t
pipe_read(struct kiocb *iocb, struct iov_iter *to)
{
size_t total_len = iov_iter_count(to);
struct file *filp = iocb->ki_filp;
struct pipe_inode_info *pipe = filp->private_data;
int do_wakeup;
ssize_t ret;
/* Null read succeeds. */
if (unlikely(total_len == 0))
return 0;
do_wakeup = 0;
ret = 0;
__pipe_lock(pipe);
for (;;) {
int bufs = pipe->nrbufs;
if (bufs) {
int curbuf = pipe->curbuf;
struct pipe_buffer *buf = pipe->bufs + curbuf;
size_t chars = buf->len;
size_t written;
int error;
if (chars > total_len)
chars = total_len;
error = pipe_buf_confirm(pipe, buf);
if (error) {
if (!ret)
ret = error;
break;
}
written = copy_page_to_iter(buf->page, buf->offset, chars, to);
if (unlikely(written < chars)) {
if (!ret)
ret = -EFAULT;
break;
}
ret += chars;
buf->offset += chars;
buf->len -= chars;
/* Was it a packet buffer? Clean up and exit */
if (buf->flags & PIPE_BUF_FLAG_PACKET) {
total_len = chars;
buf->len = 0;
}
if (!buf->len) {
pipe_buf_release(pipe, buf);
curbuf = (curbuf + 1) & (pipe->buffers - 1);
pipe->curbuf = curbuf;
pipe->nrbufs = --bufs;
do_wakeup = 1;
}
total_len -= chars;
if (!total_len)
break; /* common path: read succeeded */
}
if (bufs) /* More to do? */
continue;
if (!pipe->writers)
break;
if (!pipe->waiting_writers) {
/* syscall merging: Usually we must not sleep
* if O_NONBLOCK is set, or if we got some data.
* But if a writer sleeps in kernel space, then
* we can wait for that data without violating POSIX.
*/
if (ret)
break;
if (filp->f_flags & O_NONBLOCK) {
ret = -EAGAIN;
break;
}
}
if (signal_pending(current)) {
if (!ret)
ret = -ERESTARTSYS;
break;
}
if (do_wakeup) {
wake_up_interruptible_sync_poll(&pipe->wait, POLLOUT | POLLWRNORM);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
pipe_wait(pipe);
}
__pipe_unlock(pipe);
/* Signal writers asynchronously that there is more room. */
if (do_wakeup) {
wake_up_interruptible_sync_poll(&pipe->wait, POLLOUT | POLLWRNORM);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
if (ret > 0)
file_accessed(filp);
return ret;
}
static ssize_t
pipe_write(struct kiocb *iocb, struct iov_iter *from)
{
struct file *filp = iocb->ki_filp;
struct pipe_inode_info *pipe = filp->private_data;
ssize_t ret = 0;
int do_wakeup = 0;
size_t total_len = iov_iter_count(from);
ssize_t chars;
/* Null write succeeds. */
if (unlikely(total_len == 0))
return 0;
__pipe_lock(pipe);
if (!pipe->readers) {
send_sig(SIGPIPE, current, 0);
ret = -EPIPE;
goto out;
}
/* We try to merge small writes */
chars = total_len & (PAGE_SIZE-1); /* size of the last buffer */
if (pipe->nrbufs && chars != 0) {
int lastbuf = (pipe->curbuf + pipe->nrbufs - 1) &
(pipe->buffers - 1);
struct pipe_buffer *buf = pipe->bufs + lastbuf;
int offset = buf->offset + buf->len;
if (buf->ops->can_merge && offset + chars <= PAGE_SIZE) {
ret = pipe_buf_confirm(pipe, buf);
if (ret)
goto out;
ret = copy_page_from_iter(buf->page, offset, chars, from);
if (unlikely(ret < chars)) {
ret = -EFAULT;
goto out;
}
do_wakeup = 1;
buf->len += ret;
if (!iov_iter_count(from))
goto out;
}
}
for (;;) {
int bufs;
if (!pipe->readers) {
send_sig(SIGPIPE, current, 0);
if (!ret)
ret = -EPIPE;
break;
}
bufs = pipe->nrbufs;
if (bufs < pipe->buffers) {
int newbuf = (pipe->curbuf + bufs) & (pipe->buffers-1);
struct pipe_buffer *buf = pipe->bufs + newbuf;
struct page *page = pipe->tmp_page;
int copied;
if (!page) {
page = alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT);
if (unlikely(!page)) {
ret = ret ? : -ENOMEM;
break;
}
pipe->tmp_page = page;
}
/* Always wake up, even if the copy fails. Otherwise
* we lock up (O_NONBLOCK-)readers that sleep due to
* syscall merging.
* FIXME! Is this really true?
*/
do_wakeup = 1;
copied = copy_page_from_iter(page, 0, PAGE_SIZE, from);
if (unlikely(copied < PAGE_SIZE && iov_iter_count(from))) {
if (!ret)
ret = -EFAULT;
break;
}
ret += copied;
/* Insert it into the buffer array */
buf->page = page;
buf->ops = &anon_pipe_buf_ops;
buf->offset = 0;
buf->len = copied;
buf->flags = 0;
if (is_packetized(filp)) {
buf->ops = &packet_pipe_buf_ops;
buf->flags = PIPE_BUF_FLAG_PACKET;
}
pipe->nrbufs = ++bufs;
pipe->tmp_page = NULL;
if (!iov_iter_count(from))
break;
}
if (bufs < pipe->buffers)
continue;
if (filp->f_flags & O_NONBLOCK) {
if (!ret)
ret = -EAGAIN;
break;
}
if (signal_pending(current)) {
if (!ret)
ret = -ERESTARTSYS;
break;
}
if (do_wakeup) {
wake_up_interruptible_sync_poll(&pipe->wait, POLLIN | POLLRDNORM);
kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
do_wakeup = 0;
}
pipe->waiting_writers++;
pipe_wait(pipe);
pipe->waiting_writers--;
}
out:
__pipe_unlock(pipe);
if (do_wakeup) {
wake_up_interruptible_sync_poll(&pipe->wait, POLLIN | POLLRDNORM);
kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
}
if (ret > 0 && sb_start_write_trylock(file_inode(filp)->i_sb)) {
int err = file_update_time(filp);
if (err)
ret = err;
sb_end_write(file_inode(filp)->i_sb);
}
return ret;
}分析到这里，pipe的创建和调用已经结束了，谢谢你能内心看到最后！！！！！！！！！！！！！ |
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