Linux is a circular buffer:

/**
 *    struct pipe_buffer - a linux kernel pipe buffer
 *    @page: the page containing the data for the pipe buffer
 *    @offset: offset of data inside the @page
 *    @len: length of data inside the @page
 *    @ops: operations associated with this buffer. See @pipe_buf_operations.
 *    @flags: pipe buffer flags. See above.
 *    @private: private data owned by the ops.
 **/
struct pipe_buffer {
    struct page *page;
    unsigned int offset, len;
    const struct pipe_buf_operations *ops;
    unsigned int flags;
    unsigned long private;
};


/**
 *    struct pipe_inode_info - a linux kernel pipe
 *    @mutex: mutex protecting the whole thing
 *    @wait: reader/writer wait point in case of empty/full pipe
 *    @nrbufs: the number of non-empty pipe buffers in this pipe
 *    @buffers: total number of buffers (should be a power of 2)
 *    @curbuf: the current pipe buffer entry
 *    @tmp_page: cached released page
 *    @readers: number of current readers of this pipe
 *    @writers: number of current writers of this pipe
 *    @files: number of struct file referring this pipe (protected by ->i_lock)
 *    @waiting_writers: number of writers blocked waiting for room
 *    @r_counter: reader counter
 *    @w_counter: writer counter
 *    @fasync_readers: reader side fasync
 *    @fasync_writers: writer side fasync
 *    @bufs: the circular array of pipe buffers
 *    @user: the user who created this pipe
 **/
struct pipe_inode_info {
    struct mutex mutex;
    wait_queue_head_t wait;
    unsigned int nrbufs, curbuf, buffers;
    unsigned int readers;
    unsigned int writers;
    unsigned int files;
    unsigned int waiting_writers;
    unsigned int r_counter;
    unsigned int w_counter;
    struct page *tmp_page;
    struct fasync_struct *fasync_readers;
    struct fasync_struct *fasync_writers;
    struct pipe_buffer *bufs;
    struct user_struct *user;
};

curbuf is the subscript of the current cache, and each buffer contains the location where offset and len record data is written, and this information needs to be modified when reading and writing.

if two processes read or write at the same time, it is necessary to cause data conflicts, so the kernel locks the pipes (mutex), in pipe_inode_info is therefore half-duplex.

struct pipe {
  struct spinlock lock;
  char data[PIPESIZE];
  uint nread;     // number of bytes read
  uint nwrite;    // number of bytes written
  int readopen;   // read fd is still open
  int writeopen;  // write fd is still open
};

directly a continuous piece of memory data , two digits nread and nwrite are used to record the number of read and write. The read and write position on data is obtained by PIPESIZE, and protected by spin lock. If there is no lock, both processes can write data to data and modify nwrite at the same time.