对于 epoll 实现原理的理解

struct epitem {    union {        /* RB tree node links this structure to the eventpoll RB tree */        struct rb_node rbn;        /* Used to free the struct epitem */        struct rcu_head rcu;    };
    /* List header used to link this structure to the eventpoll ready list */    struct list_head rdllink;
    /*     * Works together "struct eventpoll"->ovflist in keeping the     * single linked chain of items.     */    struct epitem *next;
    /* The file descriptor information this item refers to */    struct epoll_filefd ffd;//sockfd
    /* List containing poll wait queues */    struct eppoll_entry *pwqlist;
    /* The "container" of this item */    struct eventpoll *ep;
    /* List header used to link this item to the "struct file" items list */    struct hlist_node fllink;
    /* wakeup_source used when EPOLLWAKEUP is set */    struct wakeup_source __rcu *ws;
    /* The structure that describe the interested events and the source fd */    struct epoll_event event;};
struct eventpoll {
    struct mutex mtx;
    /* Wait queue used by sys_epoll_wait() */    wait_queue_head_t wq;
    /* Wait queue used by file->poll() */    wait_queue_head_t poll_wait;
    /* List of ready file descriptors */    struct list_head rdllist;
    /* Lock which protects rdllist and ovflist */    rwlock_t lock;
    /* RB tree root used to store monitored fd structs */    struct rb_root_cached 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.     */    struct epitem *ovflist;
    /* wakeup_source used when ep_scan_ready_list is running */    struct wakeup_source *ws;
    /* The user that created the eventpoll descriptor */    struct user_struct *user;
    struct file *file;
    /* used to optimize loop detection check */    u64 gen;    struct hlist_head refs;
#ifdef CONFIG_NET_RX_BUSY_POLL    /* used to track busy poll napi_id */    unsigned int napi_id;#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC    /* tracks wakeup nests for lockdep validation */    u8 nests;#endif};