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    mod context;
    mod manager;
    mod pid;
    mod processor;
    mod signal;
    mod switch;
    mod action;
    #[allow(clippy::module_inception)]
    mod task;

    use crate::fs::{open_file, OpenFlags};
    use alloc::sync::Arc;
    pub use context::TaskContext;
    use lazy_static::*;
    use manager::fetch_task;
    use manager::remove_from_pid2task;
    use switch::__switch;
    use task::{TaskControlBlock, TaskStatus};

    pub use manager::{add_task, pid2task};
    pub use pid::{pid_alloc, KernelStack, PidHandle};
    pub use processor::{
        current_task, current_trap_cx, current_user_token, run_tasks, schedule, take_current_task,
    };
    pub use signal::{SignalFlags, MAX_SIG};
    pub use action::{SignalAction, SignalActions};

    pub fn suspend_current_and_run_next() {
        // There must be an application running.
        let task = take_current_task().unwrap();

        // ---- access current TCB exclusively
        let mut task_inner = task.inner_exclusive_access();
        let task_cx_ptr = &mut task_inner.task_cx as *mut TaskContext;
        // Change status to Ready
        task_inner.task_status = TaskStatus::Ready;
        drop(task_inner);
        // ---- release current PCB

        // push back to ready queue.
        add_task(task);
        // jump to scheduling cycle
        schedule(task_cx_ptr);
    }

    pub fn exit_current_and_run_next(exit_code: i32) {
        // take from Processor
        let task = take_current_task().unwrap();
        // remove from pid2task
        remove_from_pid2task(task.getpid());
        // **** access current TCB exclusively
        let mut inner = task.inner_exclusive_access();
        // Change status to Zombie
        inner.task_status = TaskStatus::Zombie;
        // Record exit code
        inner.exit_code = exit_code;
        // do not move to its parent but under initproc

        // ++++++ access initproc TCB exclusively
        {
            let mut initproc_inner = INITPROC.inner_exclusive_access();
            for child in inner.children.iter() {
                child.inner_exclusive_access().parent = Some(Arc::downgrade(&INITPROC));
                initproc_inner.children.push(child.clone());
            }
        }
        // ++++++ release parent PCB

        inner.children.clear();
        // deallocate user space
        inner.memory_set.recycle_data_pages();
        // drop file descriptors
        inner.fd_table.clear();
        drop(inner);
        // **** release current PCB
        // drop task manually to maintain rc correctly
        drop(task);
        // we do not have to save task context
        let mut _unused = TaskContext::zero_init();
        schedule(&mut _unused as *mut _);
    }

    lazy_static! {
        pub static ref INITPROC: Arc<TaskControlBlock> = Arc::new({
            let inode = open_file("initproc", OpenFlags::RDONLY).unwrap();
            let v = inode.read_all();
            TaskControlBlock::new(v.as_slice())
        });
    }

    pub fn add_initproc() {
        add_task(INITPROC.clone());
    }

    pub fn check_signals_error_of_current() -> Option<(i32, &'static str)> {
        let task = current_task().unwrap();
        let task_inner = task.inner_exclusive_access();
        task_inner.signals.check_error()
    }

    pub fn current_add_signal(signal: SignalFlags) {
        let task = current_task().unwrap();
        let mut task_inner = task.inner_exclusive_access();
        task_inner.signals |= signal;
    }

    fn call_kernel_signal_handler(signal: SignalFlags) {
        let task = current_task().unwrap();
        let mut task_inner = task.inner_exclusive_access();
        match signal {
            SignalFlags::SIGSTOP => {
                task_inner.frozen = true;
                task_inner.signals ^= SignalFlags::SIGSTOP;
            }
            SignalFlags::SIGCONT => {
                if task_inner.signals.contains(SignalFlags::SIGCONT) {
                    task_inner.signals ^= SignalFlags::SIGCONT;
                    task_inner.frozen = false;
                }
            }
            _ => {
                task_inner.killed = true;
            }
        }
    }

    fn call_user_signal_handler(sig: usize, signal: SignalFlags) {
        let task = current_task().unwrap();
        let mut task_inner = task.inner_exclusive_access();

        let handler = task_inner.signal_actions.table[sig].handler;
        // change current mask
        task_inner.signal_mask = task_inner.signal_actions.table[sig].mask;
        // handle flag
        task_inner.handling_sig = sig as isize;
        task_inner.signals ^= signal;

        // backup trapframe
        let mut trap_ctx = task_inner.get_trap_cx();
        task_inner.trap_ctx_backup = Some(*trap_ctx);

        // modify trapframe
        trap_ctx.sepc = handler;

        // put args (a0)
        trap_ctx.x[10] = sig;
    }

    fn check_pending_signals() {
        for sig in 0..(MAX_SIG + 1) {
            let task = current_task().unwrap();
            let task_inner = task.inner_exclusive_access();
            let signal = SignalFlags::from_bits(1 << sig).unwrap();
            if task_inner.signals.contains(signal) && (!task_inner.signal_mask.contains(signal)) {
                if task_inner.handling_sig == -1 {
                    drop(task_inner);
                    drop(task);
                    if signal == SignalFlags::SIGKILL || signal == SignalFlags::SIGSTOP ||
                        signal == SignalFlags::SIGCONT || signal == SignalFlags::SIGDEF {
                            // signal is a kernel signal
                            call_kernel_signal_handler(signal);
                    } else {
                        // signal is a user signal
                        call_user_signal_handler(sig, signal);
                        return;
                    }
                } else {
                    if !task_inner.signal_actions.table[task_inner.handling_sig as usize].mask.contains(signal) {
                        drop(task_inner);
                        drop(task);
                        if signal == SignalFlags::SIGKILL || signal == SignalFlags::SIGSTOP ||
                            signal == SignalFlags::SIGCONT || signal == SignalFlags::SIGDEF {
                                // signal is a kernel signal
                                call_kernel_signal_handler(signal);
                        } else {
                            // signal is a user signal
                            call_user_signal_handler(sig, signal);
                            return;
                        }
                    }
                }
            }
        }
    }

    pub fn handle_signals() {
        check_pending_signals();
        loop {
            let task = current_task().unwrap();
            let task_inner = task.inner_exclusive_access();
            let frozen_flag = task_inner.frozen;
            let killed_flag = task_inner.killed;
            drop(task_inner);
            drop(task);
            if (!frozen_flag) || killed_flag {
                break;
            }
            check_pending_signals();
            suspend_current_and_run_next()
        }
    }