/*
 * Implement CPU time clocks for the POSIX clock interface.
 */

#include <linux/sched.h>
#include <linux/posix-timers.h>
#include <linux/errno.h>
#include <linux/math64.h>
#include <asm/uaccess.h>
#include <linux/kernel_stat.h>
#include <trace/events/timer.h>
#include <linux/random.h>
#include <linux/tick.h>
#include <linux/workqueue.h>

/*
 * Called after updating RLIMIT_CPU to run cpu timer and update
 * tsk->signal->cputime_expires expiration cache if necessary. Needs
 * siglock protection since other code may update expiration cache as
 * well.
 */
void update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new)
{
        cputime_t cputime = secs_to_cputime(rlim_new);

        spin_lock_irq(&task->sighand->siglock);
        set_process_cpu_timer(task, CPUCLOCK_PROF, &cputime, NULL);
        spin_unlock_irq(&task->sighand->siglock);
}

static int check_clock(const clockid_t which_clock)
{
        int error = 0;
        struct task_struct *p;
        const pid_t pid = CPUCLOCK_PID(which_clock);

        if (CPUCLOCK_WHICH(which_clock) >= CPUCLOCK_MAX)
                return -EINVAL;

        if (pid == 0)
                return 0;

        rcu_read_lock();
        p = find_task_by_vpid(pid);
        if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
                   same_thread_group(p, current) : has_group_leader_pid(p))) {
                error = -EINVAL;
        }
        rcu_read_unlock();

        return error;
}

static inline unsigned long long
timespec_to_sample(const clockid_t which_clock, const struct timespec *tp)
{
        unsigned long long ret;

        ret = 0;                /* high half always zero when .cpu used */
        if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED) {
                ret = (unsigned long long)tp->tv_sec * NSEC_PER_SEC + tp->tv_nsec;
        } else {
                ret = cputime_to_expires(timespec_to_cputime(tp));
        }
        return ret;
}

static void sample_to_timespec(const clockid_t which_clock,
                               unsigned long long expires,
                               struct timespec *tp)
{
        if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED)
                *tp = ns_to_timespec(expires);
        else
                cputime_to_timespec((__force cputime_t)expires, tp);
}

/*
 * Update expiry time from increment, and increase overrun count,
 * given the current clock sample.
 */
static void bump_cpu_timer(struct k_itimer *timer,
                           unsigned long long now)
{
        int i;
        unsigned long long delta, incr;

        if (timer->it.cpu.incr == 0)
                return;

        if (now < timer->it.cpu.expires)
                return;

        incr = timer->it.cpu.incr;
        delta = now + incr - timer->it.cpu.expires;

        /* Don't use (incr*2 < delta), incr*2 might overflow. */
        for (i = 0; incr < delta - incr; i++)
                incr = incr << 1;

        for (; i >= 0; incr >>= 1, i--) {
                if (delta < incr)
                        continue;

                timer->it.cpu.expires += incr;
                timer->it_overrun += 1 << i;
                delta -= incr;
        }
}

/**
 * task_cputime_zero - Check a task_cputime struct for all zero fields.
 *
 * @cputime:    The struct to compare.
 *
 * Checks @cputime to see if all fields are zero.  Returns true if all fields
 * are zero, false if any field is nonzero.
 */
static inline int task_cputime_zero(const struct task_cputime *cputime)
{
        if (!cputime->utime && !cputime->stime && !cputime->sum_exec_runtime)
                return 1;
        return 0;
}

static inline unsigned long long prof_ticks(struct task_struct *p)
{
        cputime_t utime, stime;

        task_cputime(p, &utime, &stime);

        return cputime_to_expires(utime + stime);
}
static inline unsigned long long virt_ticks(struct task_struct *p)
{
        cputime_t utime;

        task_cputime(p, &utime, NULL);

        return cputime_to_expires(utime);
}

static int
posix_cpu_clock_getres(const clockid_t which_clock, struct timespec *tp)
{
        int error = check_clock(which_clock);
        if (!error) {
                tp->tv_sec = 0;
                tp->tv_nsec = ((NSEC_PER_SEC + HZ - 1) / HZ);
                if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED) {
                        /*
                         * If sched_clock is using a cycle counter, we
                         * don't have any idea of its true resolution
                         * exported, but it is much more than 1s/HZ.
                         */
                        tp->tv_nsec = 1;
                }
        }
        return error;
}

static int
posix_cpu_clock_set(const clockid_t which_clock, const struct timespec *tp)
{
        /*
         * You can never reset a CPU clock, but we check for other errors
         * in the call before failing with EPERM.
         */
        int error = check_clock(which_clock);
        if (error == 0) {
                error = -EPERM;
        }
        return error;
}


/*
 * Sample a per-thread clock for the given task.
 */
static int cpu_clock_sample(const clockid_t which_clock, struct task_struct *p,
                            unsigned long long *sample)
{
        switch (CPUCLOCK_WHICH(which_clock)) {
        default:
                return -EINVAL;
        case CPUCLOCK_PROF:
                *sample = prof_ticks(p);
                break;
        case CPUCLOCK_VIRT:
                *sample = virt_ticks(p);
                break;
        case CPUCLOCK_SCHED:
                *sample = task_sched_runtime(p);
                break;
        }
        return 0;
}

/*
 * Set cputime to sum_cputime if sum_cputime > cputime. Use cmpxchg
 * to avoid race conditions with concurrent updates to cputime.
 */
static inline void __update_gt_cputime(atomic64_t *cputime, u64 sum_cputime)
{
        u64 curr_cputime;
retry:
        curr_cputime = atomic64_read(cputime);
        if (sum_cputime > curr_cputime) {
                if (atomic64_cmpxchg(cputime, curr_cputime, sum_cputime) != curr_cputime)
                        goto retry;
        }
}

static void update_gt_cputime(struct task_cputime_atomic *cputime_atomic, struct task_cputime *sum)
{
        __update_gt_cputime(&cputime_atomic->utime, sum->utime);
        __update_gt_cputime(&cputime_atomic->stime, sum->stime);
        __update_gt_cputime(&cputime_atomic->sum_exec_runtime, sum->sum_exec_runtime);
}

/* Sample task_cputime_atomic values in "atomic_timers", store results in "times". */
static inline void sample_cputime_atomic(struct task_cputime *times,
                                         struct task_cputime_atomic *atomic_times)
{
        times->utime = atomic64_read(&atomic_times->utime);
        times->stime = atomic64_read(&atomic_times->stime);
        times->sum_exec_runtime = atomic64_read(&atomic_times->sum_exec_runtime);
}

void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
{
        struct thread_group_cputimer *cputimer = &tsk->signal->cputimer;
        struct task_cputime sum;

        /* Check if cputimer isn't running. This is accessed without locking. */
        if (!READ_ONCE(cputimer->running)) {
                /*
                 * The POSIX timer interface allows for absolute time expiry
                 * values through the TIMER_ABSTIME flag, therefore we have
                 * to synchronize the timer to the clock every time we start it.
                 */
                thread_group_cputime(tsk, &sum);
                update_gt_cputime(&cputimer->cputime_atomic, &sum);

                /*
                 * We're setting cputimer->running without a lock. Ensure
                 * this only gets written to in one operation. We set
                 * running after update_gt_cputime() as a small optimization,
                 * but barriers are not required because update_gt_cputime()
                 * can handle concurrent updates.
                 */
                WRITE_ONCE(cputimer->running, 1);
        }
        sample_cputime_atomic(times, &cputimer->cputime_atomic);
}

/*
 * Sample a process (thread group) clock for the given group_leader task.
 * Must be called with task sighand lock held for safe while_each_thread()
 * traversal.
 */
static int cpu_clock_sample_group(const clockid_t which_clock,
                                  struct task_struct *p,
                                  unsigned long long *sample)
{
        struct task_cputime cputime;

        switch (CPUCLOCK_WHICH(which_clock)) {
        default:
                return -EINVAL;
        case CPUCLOCK_PROF:
                thread_group_cputime(p, &cputime);
                *sample = cputime_to_expires(cputime.utime + cputime.stime);
                break;
        case CPUCLOCK_VIRT:
                thread_group_cputime(p, &cputime);
                *sample = cputime_to_expires(cputime.utime);
                break;
        case CPUCLOCK_SCHED:
                thread_group_cputime(p, &cputime);
                *sample = cputime.sum_exec_runtime;
                break;
        }
        return 0;
}

static int posix_cpu_clock_get_task(struct task_struct *tsk,
                                    const clockid_t which_clock,
                                    struct timespec *tp)
{
        int err = -EINVAL;
        unsigned long long rtn;

        if (CPUCLOCK_PERTHREAD(which_clock)) {
                if (same_thread_group(tsk, current))
                        err = cpu_clock_sample(which_clock, tsk, &rtn);
        } else {
                if (tsk == current || thread_group_leader(tsk))
                        err = cpu_clock_sample_group(which_clock, tsk, &rtn);
        }

        if (!err)
                sample_to_timespec(which_clock, rtn, tp);

        return err;
}


static int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
{
        const pid_t pid = CPUCLOCK_PID(which_clock);
        int err = -EINVAL;

        if (pid == 0) {
                /*
                 * Special case constant value for our own clocks.
                 * We don't have to do any lookup to find ourselves.
                 */
                err = posix_cpu_clock_get_task(current, which_clock, tp);
        } else {
                /*
                 * Find the given PID, and validate that the caller
                 * should be able to see it.
                 */
                struct task_struct *p;
                rcu_read_lock();
                p = find_task_by_vpid(pid);
                if (p)
                        err = posix_cpu_clock_get_task(p, which_clock, tp);
                rcu_read_unlock();
        }

        return err;
}


/*
 * Validate the clockid_t for a new CPU-clock timer, and initialize the timer.
 * This is called from sys_timer_create() and do_cpu_nanosleep() with the
 * new timer already all-zeros initialized.
 */
static int posix_cpu_timer_create(struct k_itimer *new_timer)
{
        int ret = 0;
        const pid_t pid = CPUCLOCK_PID(new_timer->it_clock);
        struct task_struct *p;

        if (CPUCLOCK_WHICH(new_timer->it_clock) >= CPUCLOCK_MAX)
                return -EINVAL;

        INIT_LIST_HEAD(&new_timer->it.cpu.entry);

        rcu_read_lock();
        if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
                if (pid == 0) {
                        p = current;
                } else {
                        p = find_task_by_vpid(pid);
                        if (p && !same_thread_group(p, current))
                                p = NULL;
                }
        } else {
                if (pid == 0) {
                        p = current->group_leader;
                } else {
                        p = find_task_by_vpid(pid);
                        if (p && !has_group_leader_pid(p))
                                p = NULL;
                }
        }
        new_timer->it.cpu.task = p;
        if (p) {
                get_task_struct(p);
        } else {
                ret = -EINVAL;
        }
        rcu_read_unlock();

        return ret;
}

/*
 * Clean up a CPU-clock timer that is about to be destroyed.
 * This is called from timer deletion with the timer already locked.
 * If we return TIMER_RETRY, it's necessary to release the timer's lock
 * and try again.  (This happens when the timer is in the middle of firing.)
 */
static int posix_cpu_timer_del(struct k_itimer *timer)
{
        int ret = 0;
        unsigned long flags;
        struct sighand_struct *sighand;
        struct task_struct *p = timer->it.cpu.task;

        WARN_ON_ONCE(p == NULL);

        /*
         * Protect against sighand release/switch in exit/exec and process/
         * thread timer list entry concurrent read/writes.
         */
        sighand = lock_task_sighand(p, &flags);
        if (unlikely(sighand == NULL)) {
                /*
                 * We raced with the reaping of the task.
                 * The deletion should have cleared us off the list.
                 */
                WARN_ON_ONCE(!list_empty(&timer->it.cpu.entry));
        } else {
                if (timer->it.cpu.firing)
                        ret = TIMER_RETRY;
                else
                        list_del(&timer->it.cpu.entry);

                unlock_task_sighand(p, &flags);
        }

        if (!ret)
                put_task_struct(p);

        return ret;
}

static void cleanup_timers_list(struct list_head *head)
{
        struct cpu_timer_list *timer, *next;

        list_for_each_entry_safe(timer, next, head, entry)
                list_del_init(&timer->entry);
}

/*
 * Clean out CPU timers still ticking when a thread exited.  The task
 * pointer is cleared, and the expiry time is replaced with the residual
 * time for later timer_gettime calls to return.
 * This must be called with the siglock held.
 */
static void cleanup_timers(struct list_head *head)
{
        cleanup_timers_list(head);
        cleanup_timers_list(++head);
        cleanup_timers_list(++head);
}

/*
 * These are both called with the siglock held, when the current thread
 * is being reaped.  When the final (leader) thread in the group is reaped,
 * posix_cpu_timers_exit_group will be called after posix_cpu_timers_exit.
 */
void posix_cpu_timers_exit(struct task_struct *tsk)
{
        add_device_randomness((const void*) &tsk->se.sum_exec_runtime,
                                                sizeof(unsigned long long));
        cleanup_timers(tsk->cpu_timers);

}
void posix_cpu_timers_exit_group(struct task_struct *tsk)
{
        cleanup_timers(tsk->signal->cpu_timers);
}

static inline int expires_gt(cputime_t expires, cputime_t new_exp)
{
        return expires == 0 || expires > new_exp;
}

/*
 * Insert the timer on the appropriate list before any timers that
 * expire later.  This must be called with the sighand lock held.
 */
static void arm_timer(struct k_itimer *timer)
{
        struct task_struct *p = timer->it.cpu.task;
        struct list_head *head, *listpos;
        struct task_cputime *cputime_expires;
        struct cpu_timer_list *const nt = &timer->it.cpu;
        struct cpu_timer_list *next;

        if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
                head = p->cpu_timers;
                cputime_expires = &p->cputime_expires;
        } else {
                head = p->signal->cpu_timers;
                cputime_expires = &p->signal->cputime_expires;
        }
        head += CPUCLOCK_WHICH(timer->it_clock);

        listpos = head;
        list_for_each_entry(next, head, entry) {
                if (nt->expires < next->expires)
                        break;
                listpos = &next->entry;
        }
        list_add(&nt->entry, listpos);

        if (listpos == head) {
                unsigned long long exp = nt->expires;

                /*
                 * We are the new earliest-expiring POSIX 1.b timer, hence
                 * need to update expiration cache. Take into account that
                 * for process timers we share expiration cache with itimers
                 * and RLIMIT_CPU and for thread timers with RLIMIT_RTTIME.
                 */

                switch (CPUCLOCK_WHICH(timer->it_clock)) {
                case CPUCLOCK_PROF:
                        if (expires_gt(cputime_expires->prof_exp, expires_to_cputime(exp)))
                                cputime_expires->prof_exp = expires_to_cputime(exp);
                        break;
                case CPUCLOCK_VIRT:
                        if (expires_gt(cputime_expires->virt_exp, expires_to_cputime(exp)))
                                cputime_expires->virt_exp = expires_to_cputime(exp);
                        break;
                case CPUCLOCK_SCHED:
                        if (cputime_expires->sched_exp == 0 ||
                            cputime_expires->sched_exp > exp)
                                cputime_expires->sched_exp = exp;
                        break;
                }
        }
}

/*
 * The timer is locked, fire it and arrange for its reload.
 */
static void cpu_timer_fire(struct k_itimer *timer)
{
        if ((timer->it_sigev_notify & ~SIGEV_THREAD_ID) == SIGEV_NONE) {
                /*
                 * User don't want any signal.
                 */
                timer->it.cpu.expires = 0;
        } else if (unlikely(timer->sigq == NULL)) {
                /*
                 * This a special case for clock_nanosleep,
                 * not a normal timer from sys_timer_create.
                 */
                wake_up_process(timer->it_process);
                timer->it.cpu.expires = 0;
        } else if (timer->it.cpu.incr == 0) {
                /*
                 * One-shot timer.  Clear it as soon as it's fired.
                 */
                posix_timer_event(timer, 0);
                timer->it.cpu.expires = 0;
        } else if (posix_timer_event(timer, ++timer->it_requeue_pending)) {
                /*
                 * The signal did not get queued because the signal
                 * was ignored, so we won't get any callback to
                 * reload the timer.  But we need to keep it
                 * ticking in case the signal is deliverable next time.
                 */
                posix_cpu_timer_schedule(timer);
        }
}

/*
 * Sample a process (thread group) timer for the given group_leader task.
 * Must be called with task sighand lock held for safe while_each_thread()
 * traversal.
 */
static int cpu_timer_sample_group(const clockid_t which_clock,
                                  struct task_struct *p,
                                  unsigned long long *sample)
{
        struct task_cputime cputime;

        thread_group_cputimer(p, &cputime);
        switch (CPUCLOCK_WHICH(which_clock)) {
        default:
                return -EINVAL;
        case CPUCLOCK_PROF:
                *sample = cputime_to_expires(cputime.utime + cputime.stime);
                break;
        case CPUCLOCK_VIRT:
                *sample = cputime_to_expires(cputime.utime);
                break;
        case CPUCLOCK_SCHED:
                *sample = cputime.sum_exec_runtime;
                break;
        }
        return 0;
}

#ifdef CONFIG_NO_HZ_FULL
static void nohz_kick_work_fn(struct work_struct *work)
{
        tick_nohz_full_kick_all();
}

static DECLARE_WORK(nohz_kick_work, nohz_kick_work_fn);

/*
 * We need the IPIs to be sent from sane process context.
 * The posix cpu timers are always set with irqs disabled.
 */
static void posix_cpu_timer_kick_nohz(void)
{
        if (context_tracking_is_enabled())
                schedule_work(&nohz_kick_work);
}

bool posix_cpu_timers_can_stop_tick(struct task_struct *tsk)
{
        if (!task_cputime_zero(&tsk->cputime_expires))
                return false;

        /* Check if cputimer is running. This is accessed without locking. */
        if (READ_ONCE(tsk->signal->cputimer.running))
                return false;

        return true;
}
#else
static inline void posix_cpu_timer_kick_nohz(void) { }
#endif

/*
 * Guts of sys_timer_settime for CPU timers.
 * This is called with the timer locked and interrupts disabled.
 * If we return TIMER_RETRY, it's necessary to release the timer's lock
 * and try again.  (This happens when the timer is in the middle of firing.)
 */
static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
                               struct itimerspec *new, struct itimerspec *old)
{
        unsigned long flags;
        struct sighand_struct *sighand;
        struct task_struct *p = timer->it.cpu.task;
        unsigned long long old_expires, new_expires, old_incr, val;
        int ret;

        WARN_ON_ONCE(p == NULL);

        new_expires = timespec_to_sample(timer->it_clock, &new->it_value);

        /*
         * Protect against sighand release/switch in exit/exec and p->cpu_timers
         * and p->signal->cpu_timers read/write in arm_timer()
         */
        sighand = lock_task_sighand(p, &flags);
        /*
         * If p has just been reaped, we can no
         * longer get any information about it at all.
         */
        if (unlikely(sighand == NULL)) {
                return -ESRCH;
        }

        /*
         * Disarm any old timer after extracting its expiry time.
         */
        WARN_ON_ONCE(!irqs_disabled());

        ret = 0;
        old_incr = timer->it.cpu.incr;
        old_expires = timer->it.cpu.expires;
        if (unlikely(timer->it.cpu.firing)) {
                timer->it.cpu.firing = -1;
                ret = TIMER_RETRY;
        } else
                list_del_init(&timer->it.cpu.entry);

        /*
         * We need to sample the current value to convert the new
         * value from to relative and absolute, and to convert the
         * old value from absolute to relative.  To set a process
         * timer, we need a sample to balance the thread expiry
         * times (in arm_timer).  With an absolute time, we must
         * check if it's already passed.  In short, we need a sample.
         */
        if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
                cpu_clock_sample(timer->it_clock, p, &val);
        } else {
                cpu_timer_sample_group(timer->it_clock, p, &val);
        }

        if (old) {
                if (old_expires == 0) {
                        old->it_value.tv_sec = 0;
                        old->it_value.tv_nsec = 0;
                } else {
                        /*
                         * Update the timer in case it has
                         * overrun already.  If it has,
                         * we'll report it as having overrun
                         * and with the next reloaded timer
                         * already ticking, though we are
                         * swallowing that pending
                         * notification here to install the
                         * new setting.
                         */
                        bump_cpu_timer(timer, val);
                        if (val < timer->it.cpu.expires) {
                                old_expires = timer->it.cpu.expires - val;
                                sample_to_timespec(timer->it_clock,
                                                   old_expires,
                                                   &old->it_value);
                        } else {
                                old->it_value.tv_nsec = 1;
                                old->it_value.tv_sec = 0;
                        }
                }
        }

        if (unlikely(ret)) {
                /*
                 * We are colliding with the timer actually firing.
                 * Punt after filling in the timer's old value, and
                 * disable this firing since we are already reporting
                 * it as an overrun (thanks to bump_cpu_timer above).
                 */
                unlock_task_sighand(p, &flags);
                goto out;
        }

        if (new_expires != 0 && !(timer_flags & TIMER_ABSTIME)) {
                new_expires += val;
        }

        /*
         * Install the new expiry time (or zero).
         * For a timer with no notification action, we don't actually
         * arm the timer (we'll just fake it for timer_gettime).
         */
        timer->it.cpu.expires = new_expires;
        if (new_expires != 0 && val < new_expires) {
                arm_timer(timer);
        }

        unlock_task_sighand(p, &flags);
        /*
         * Install the new reload setting, and
         * set up the signal and overrun bookkeeping.
         */
        timer->it.cpu.incr = timespec_to_sample(timer->it_clock,
                                                &new->it_interval);

        /*
         * This acts as a modification timestamp for the timer,
         * so any automatic reload attempt will punt on seeing
         * that we have reset the timer manually.
         */
        timer->it_requeue_pending = (timer->it_requeue_pending + 2) &
                ~REQUEUE_PENDING;
        timer->it_overrun_last = 0;
        timer->it_overrun = -1;

        if (new_expires != 0 && !(val < new_expires)) {
                /*
                 * The designated time already passed, so we notify
                 * immediately, even if the thread never runs to
                 * accumulate more time on this clock.
                 */
                cpu_timer_fire(timer);
        }

        ret = 0;
 out:
        if (old) {
                sample_to_timespec(timer->it_clock,
                                   old_incr, &old->it_interval);
        }
        if (!ret)
                posix_cpu_timer_kick_nohz();
        return ret;
}

static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
{
        unsigned long long now;
        struct task_struct *p = timer->it.cpu.task;

        WARN_ON_ONCE(p == NULL);

        /*
         * Easy part: convert the reload time.
         */
        sample_to_timespec(timer->it_clock,
                           timer->it.cpu.incr, &itp->it_interval);

        if (timer->it.cpu.expires == 0) {       /* Timer not armed at all.  */
                itp->it_value.tv_sec = itp->it_value.tv_nsec = 0;
                return;
        }

        /*
         * Sample the clock to take the difference with the expiry time.
         */
        if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
                cpu_clock_sample(timer->it_clock, p, &now);
        } else {
                struct sighand_struct *sighand;
                unsigned long flags;

                /*
                 * Protect against sighand release/switch in exit/exec and
                 * also make timer sampling safe if it ends up calling
                 * thread_group_cputime().
                 */
                sighand = lock_task_sighand(p, &flags);
                if (unlikely(sighand == NULL)) {
                        /*
                         * The process has been reaped.
                         * We can't even collect a sample any more.
                         * Call the timer disarmed, nothing else to do.
                         */
                        timer->it.cpu.expires = 0;
                        sample_to_timespec(timer->it_clock, timer->it.cpu.expires,
                                           &itp->it_value);
                } else {
                        cpu_timer_sample_group(timer->it_clock, p, &now);
                        unlock_task_sighand(p, &flags);
                }
        }

        if (now < timer->it.cpu.expires) {
                sample_to_timespec(timer->it_clock,
                                   timer->it.cpu.expires - now,
                                   &itp->it_value);
        } else {
                /*
                 * The timer should have expired already, but the firing
                 * hasn't taken place yet.  Say it's just about to expire.
                 */
                itp->it_value.tv_nsec = 1;
                itp->it_value.tv_sec = 0;
        }
}

static unsigned long long
check_timers_list(struct list_head *timers,
                  struct list_head *firing,
                  unsigned long long curr)
{
        int maxfire = 20;

        while (!list_empty(timers)) {
                struct cpu_timer_list *t;

                t = list_first_entry(timers, struct cpu_timer_list, entry);

                if (!--maxfire || curr < t->expires)
                        return t->expires;

                t->firing = 1;
                list_move_tail(&t->entry, firing);
        }

        return 0;
}

/*
 * Check for any per-thread CPU timers that have fired and move them off
 * the tsk->cpu_timers[N] list onto the firing list.  Here we update the
 * tsk->it_*_expires values to reflect the remaining thread CPU timers.
 */
static void check_thread_timers(struct task_struct *tsk,
                                struct list_head *firing)
{
        struct list_head *timers = tsk->cpu_timers;
        struct signal_struct *const sig = tsk->signal;
        struct task_cputime *tsk_expires = &tsk->cputime_expires;
        unsigned long long expires;
        unsigned long soft;

        expires = check_timers_list(timers, firing, prof_ticks(tsk));
        tsk_expires->prof_exp = expires_to_cputime(expires);

        expires = check_timers_list(++timers, firing, virt_ticks(tsk));
        tsk_expires->virt_exp = expires_to_cputime(expires);

        tsk_expires->sched_exp = check_timers_list(++timers, firing,
                                                   tsk->se.sum_exec_runtime);

        /*
         * Check for the special case thread timers.
         */
        soft = READ_ONCE(sig->rlim[RLIMIT_RTTIME].rlim_cur);
        if (soft != RLIM_INFINITY) {
                unsigned long hard =
                        READ_ONCE(sig->rlim[RLIMIT_RTTIME].rlim_max);

                if (hard != RLIM_INFINITY &&
                    tsk->rt.timeout > DIV_ROUND_UP(hard, USEC_PER_SEC/HZ)) {
                        /*
                         * At the hard limit, we just die.
                         * No need to calculate anything else now.
                         */
                        __group_send_sig_info(SIGKILL, SEND_SIG_PRIV, tsk);
                        return;
                }
                if (tsk->rt.timeout > DIV_ROUND_UP(soft, USEC_PER_SEC/HZ)) {
                        /*
                         * At the soft limit, send a SIGXCPU every second.
                         */
                        if (soft < hard) {
                                soft += USEC_PER_SEC;
                                sig->rlim[RLIMIT_RTTIME].rlim_cur = soft;
                        }
                        printk(KERN_INFO
                                "RT Watchdog Timeout: %s[%d]\n",
                                tsk->comm, task_pid_nr(tsk));
                        __group_send_sig_info(SIGXCPU, SEND_SIG_PRIV, tsk);
                }
        }
}

static inline void stop_process_timers(struct signal_struct *sig)
{
        struct thread_group_cputimer *cputimer = &sig->cputimer;

        /* Turn off cputimer->running. This is done without locking. */
        WRITE_ONCE(cputimer->running, 0);
}

static u32 onecputick;

static void check_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
                             unsigned long long *expires,
                             unsigned long long cur_time, int signo)
{
        if (!it->expires)
                return;

        if (cur_time >= it->expires) {
                if (it->incr) {
                        it->expires += it->incr;
                        it->error += it->incr_error;
                        if (it->error >= onecputick) {
                                it->expires -= cputime_one_jiffy;
                                it->error -= onecputick;
                        }
                } else {
                        it->expires = 0;
                }

                trace_itimer_expire(signo == SIGPROF ?
                                    ITIMER_PROF : ITIMER_VIRTUAL,
                                    tsk->signal->leader_pid, cur_time);
                __group_send_sig_info(signo, SEND_SIG_PRIV, tsk);
        }

        if (it->expires && (!*expires || it->expires < *expires)) {
                *expires = it->expires;
        }
}

/*
 * Check for any per-thread CPU timers that have fired and move them
 * off the tsk->*_timers list onto the firing list.  Per-thread timers
 * have already been taken off.
 */
static void check_process_timers(struct task_struct *tsk,
                                 struct list_head *firing)
{
        struct signal_struct *const sig = tsk->signal;
        unsigned long long utime, ptime, virt_expires, prof_expires;
        unsigned long long sum_sched_runtime, sched_expires;
        struct list_head *timers = sig->cpu_timers;
        struct task_cputime cputime;
        unsigned long soft;

        /*
         * Collect the current process totals.
         */
        thread_group_cputimer(tsk, &cputime);
        utime = cputime_to_expires(cputime.utime);
        ptime = utime + cputime_to_expires(cputime.stime);
        sum_sched_runtime = cputime.sum_exec_runtime;

        prof_expires = check_timers_list(timers, firing, ptime);
        virt_expires = check_timers_list(++timers, firing, utime);
        sched_expires = check_timers_list(++timers, firing, sum_sched_runtime);

        /*
         * Check for the special case process timers.
         */
        check_cpu_itimer(tsk, &sig->it[CPUCLOCK_PROF], &prof_expires, ptime,
                         SIGPROF);
        check_cpu_itimer(tsk, &sig->it[CPUCLOCK_VIRT], &virt_expires, utime,
                         SIGVTALRM);
        soft = READ_ONCE(sig->rlim[RLIMIT_CPU].rlim_cur);
        if (soft != RLIM_INFINITY) {
                unsigned long psecs = cputime_to_secs(ptime);
                unsigned long hard =
                        READ_ONCE(sig->rlim[RLIMIT_CPU].rlim_max);
                cputime_t x;
                if (psecs >= hard) {
                        /*
                         * At the hard limit, we just die.
                         * No need to calculate anything else now.
                         */
                        __group_send_sig_info(SIGKILL, SEND_SIG_PRIV, tsk);
                        return;
                }
                if (psecs >= soft) {
                        /*
                         * At the soft limit, send a SIGXCPU every second.
                         */

                        __group_send_sig_info(SIGXCPU, SEND_SIG_PRIV, tsk);
                        if (soft < hard) {
                                soft++;
                                sig->rlim[RLIMIT_CPU].rlim_cur = soft;
                        }
                }
                x = secs_to_cputime(soft);
                if (!prof_expires || x < prof_expires) {
                        prof_expires = x;
                }
        }

        sig->cputime_expires.prof_exp = expires_to_cputime(prof_expires);
        sig->cputime_expires.virt_exp = expires_to_cputime(virt_expires);
        sig->cputime_expires.sched_exp = sched_expires;
        if (task_cputime_zero(&sig->cputime_expires))
                stop_process_timers(sig);
}

/*
 * This is called from the signal code (via do_schedule_next_timer)
 * when the last timer signal was delivered and we have to reload the timer.
 */
void posix_cpu_timer_schedule(struct k_itimer *timer)
{
        struct sighand_struct *sighand;
        unsigned long flags;
        struct task_struct *p = timer->it.cpu.task;
        unsigned long long now;

        WARN_ON_ONCE(p == NULL);

        /*
         * Fetch the current sample and update the timer's expiry time.
         */
        if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
                cpu_clock_sample(timer->it_clock, p, &now);
                bump_cpu_timer(timer, now);
                if (unlikely(p->exit_state))
                        goto out;

                /* Protect timer list r/w in arm_timer() */
                sighand = lock_task_sighand(p, &flags);
                if (!sighand)
                        goto out;
        } else {
                /*
                 * Protect arm_timer() and timer sampling in case of call to
                 * thread_group_cputime().
                 */
                sighand = lock_task_sighand(p, &flags);
                if (unlikely(sighand == NULL)) {
                        /*
                         * The process has been reaped.
                         * We can't even collect a sample any more.
                         */
                        timer->it.cpu.expires = 0;
                        goto out;
                } else if (unlikely(p->exit_state) && thread_group_empty(p)) {
                        unlock_task_sighand(p, &flags);
                        /* Optimizations: if the process is dying, no need to rearm */
                        goto out;
                }
                cpu_timer_sample_group(timer->it_clock, p, &now);
                bump_cpu_timer(timer, now);
                /* Leave the sighand locked for the call below.  */
        }

        /*
         * Now re-arm for the new expiry time.
         */
        WARN_ON_ONCE(!irqs_disabled());
        arm_timer(timer);
        unlock_task_sighand(p, &flags);

        /* Kick full dynticks CPUs in case they need to tick on the new timer */
        posix_cpu_timer_kick_nohz();
out:
        timer->it_overrun_last = timer->it_overrun;
        timer->it_overrun = -1;
        ++timer->it_requeue_pending;
}

/**
 * task_cputime_expired - Compare two task_cputime entities.
 *
 * @sample:     The task_cputime structure to be checked for expiration.
 * @expires:    Expiration times, against which @sample will be checked.
 *
 * Checks @sample against @expires to see if any field of @sample has expired.
 * Returns true if any field of the former is greater than the corresponding
 * field of the latter if the latter field is set.  Otherwise returns false.
 */
static inline int task_cputime_expired(const struct task_cputime *sample,
                                        const struct task_cputime *expires)
{
        if (expires->utime && sample->utime >= expires->utime)
                return 1;
        if (expires->stime && sample->utime + sample->stime >= expires->stime)
                return 1;
        if (expires->sum_exec_runtime != 0 &&
            sample->sum_exec_runtime >= expires->sum_exec_runtime)
                return 1;
        return 0;
}

/**
 * fastpath_timer_check - POSIX CPU timers fast path.
 *
 * @tsk:        The task (thread) being checked.
 *
 * Check the task and thread group timers.  If both are zero (there are no
 * timers set) return false.  Otherwise snapshot the task and thread group
 * timers and compare them with the corresponding expiration times.  Return
 * true if a timer has expired, else return false.
 */
static inline int fastpath_timer_check(struct task_struct *tsk)
{
        struct signal_struct *sig;
        cputime_t utime, stime;

        task_cputime(tsk, &utime, &stime);

        if (!task_cputime_zero(&tsk->cputime_expires)) {
                struct task_cputime task_sample = {
                        .utime = utime,
                        .stime = stime,
                        .sum_exec_runtime = tsk->se.sum_exec_runtime
                };

                if (task_cputime_expired(&task_sample, &tsk->cputime_expires))
                        return 1;
        }

        sig = tsk->signal;
        /* Check if cputimer is running. This is accessed without locking. */
        if (READ_ONCE(sig->cputimer.running)) {
                struct task_cputime group_sample;

                sample_cputime_atomic(&group_sample, &sig->cputimer.cputime_atomic);

                if (task_cputime_expired(&group_sample, &sig->cputime_expires))
                        return 1;
        }

        return 0;
}

/*
 * This is called from the timer interrupt handler.  The irq handler has
 * already updated our counts.  We need to check if any timers fire now.
 * Interrupts are disabled.
 */
void run_posix_cpu_timers(struct task_struct *tsk)
{
        LIST_HEAD(firing);
        struct k_itimer *timer, *next;
        unsigned long flags;

        WARN_ON_ONCE(!irqs_disabled());

        /*
         * The fast path checks that there are no expired thread or thread
         * group timers.  If that's so, just return.
         */
        if (!fastpath_timer_check(tsk))
                return;

        if (!lock_task_sighand(tsk, &flags))
                return;
        /*
         * Here we take off tsk->signal->cpu_timers[N] and
         * tsk->cpu_timers[N] all the timers that are firing, and
         * put them on the firing list.
         */
        check_thread_timers(tsk, &firing);
        /*
         * If there are any active process wide timers (POSIX 1.b, itimers,
         * RLIMIT_CPU) cputimer must be running.
         */
        if (READ_ONCE(tsk->signal->cputimer.running))
                check_process_timers(tsk, &firing);

        /*
         * We must release these locks before taking any timer's lock.
         * There is a potential race with timer deletion here, as the
         * siglock now protects our private firing list.  We have set
         * the firing flag in each timer, so that a deletion attempt
         * that gets the timer lock before we do will give it up and
         * spin until we've taken care of that timer below.
         */
        unlock_task_sighand(tsk, &flags);

        /*
         * Now that all the timers on our list have the firing flag,
         * no one will touch their list entries but us.  We'll take
         * each timer's lock before clearing its firing flag, so no
         * timer call will interfere.
         */
        list_for_each_entry_safe(timer, next, &firing, it.cpu.entry) {
                int cpu_firing;

                spin_lock(&timer->it_lock);
                list_del_init(&timer->it.cpu.entry);
                cpu_firing = timer->it.cpu.firing;
                timer->it.cpu.firing = 0;
                /*
                 * The firing flag is -1 if we collided with a reset
                 * of the timer, which already reported this
                 * almost-firing as an overrun.  So don't generate an event.
                 */
                if (likely(cpu_firing >= 0))
                        cpu_timer_fire(timer);
                spin_unlock(&timer->it_lock);
        }
}

/*
 * Set one of the process-wide special case CPU timers or RLIMIT_CPU.
 * The tsk->sighand->siglock must be held by the caller.
 */
void set_process_cpu_timer(struct task_struct *tsk, unsigned int clock_idx,
                           cputime_t *newval, cputime_t *oldval)
{
        unsigned long long now;

        WARN_ON_ONCE(clock_idx == CPUCLOCK_SCHED);
        cpu_timer_sample_group(clock_idx, tsk, &now);

        if (oldval) {
                /*
                 * We are setting itimer. The *oldval is absolute and we update
                 * it to be relative, *newval argument is relative and we update
                 * it to be absolute.
                 */
                if (*oldval) {
                        if (*oldval <= now) {
                                /* Just about to fire. */
                                *oldval = cputime_one_jiffy;
                        } else {
                                *oldval -= now;
                        }
                }

                if (!*newval)
                        goto out;
                *newval += now;
        }

        /*
         * Update expiration cache if we are the earliest timer, or eventually
         * RLIMIT_CPU limit is earlier than prof_exp cpu timer expire.
         */
        switch (clock_idx) {
        case CPUCLOCK_PROF:
                if (expires_gt(tsk->signal->cputime_expires.prof_exp, *newval))
                        tsk->signal->cputime_expires.prof_exp = *newval;
                break;
        case CPUCLOCK_VIRT:
                if (expires_gt(tsk->signal->cputime_expires.virt_exp, *newval))
                        tsk->signal->cputime_expires.virt_exp = *newval;
                break;
        }
out:
        posix_cpu_timer_kick_nohz();
}

static int do_cpu_nanosleep(const clockid_t which_clock, int flags,
                            struct timespec *rqtp, struct itimerspec *it)
{
        struct k_itimer timer;
        int error;

        /*
         * Set up a temporary timer and then wait for it to go off.
         */
        memset(&timer, 0, sizeof timer);
        spin_lock_init(&timer.it_lock);
        timer.it_clock = which_clock;
        timer.it_overrun = -1;
        error = posix_cpu_timer_create(&timer);
        timer.it_process = current;
        if (!error) {
                static struct itimerspec zero_it;

                memset(it, 0, sizeof *it);
                it->it_value = *rqtp;

                spin_lock_irq(&timer.it_lock);
                error = posix_cpu_timer_set(&timer, flags, it, NULL);
                if (error) {
                        spin_unlock_irq(&timer.it_lock);
                        return error;
                }

                while (!signal_pending(current)) {
                        if (timer.it.cpu.expires == 0) {
                                /*
                                 * Our timer fired and was reset, below
                                 * deletion can not fail.
                                 */
                                posix_cpu_timer_del(&timer);
                                spin_unlock_irq(&timer.it_lock);
                                return 0;
                        }

                        /*
                         * Block until cpu_timer_fire (or a signal) wakes us.
                         */
                        __set_current_state(TASK_INTERRUPTIBLE);
                        spin_unlock_irq(&timer.it_lock);
                        schedule();
                        spin_lock_irq(&timer.it_lock);
                }

                /*
                 * We were interrupted by a signal.
                 */
                sample_to_timespec(which_clock, timer.it.cpu.expires, rqtp);
                error = posix_cpu_timer_set(&timer, 0, &zero_it, it);
                if (!error) {
                        /*
                         * Timer is now unarmed, deletion can not fail.
                         */
                        posix_cpu_timer_del(&timer);
                }
                spin_unlock_irq(&timer.it_lock);

                while (error == TIMER_RETRY) {
                        /*
                         * We need to handle case when timer was or is in the
                         * middle of firing. In other cases we already freed
                         * resources.
                         */
                        spin_lock_irq(&timer.it_lock);
                        error = posix_cpu_timer_del(&timer);
                        spin_unlock_irq(&timer.it_lock);
                }

                if ((it->it_value.tv_sec | it->it_value.tv_nsec) == 0) {
                        /*
                         * It actually did fire already.
                         */
                        return 0;
                }

                error = -ERESTART_RESTARTBLOCK;
        }

        return error;
}

static long posix_cpu_nsleep_restart(struct restart_block *restart_block);

static int posix_cpu_nsleep(const clockid_t which_clock, int flags,
                            struct timespec *rqtp, struct timespec __user *rmtp)
{
        struct restart_block *restart_block = &current->restart_block;
        struct itimerspec it;
        int error;

        /*
         * Diagnose required errors first.
         */
        if (CPUCLOCK_PERTHREAD(which_clock) &&
            (CPUCLOCK_PID(which_clock) == 0 ||
             CPUCLOCK_PID(which_clock) == current->pid))
                return -EINVAL;

        error = do_cpu_nanosleep(which_clock, flags, rqtp, &it);

        if (error == -ERESTART_RESTARTBLOCK) {

                if (flags & TIMER_ABSTIME)
                        return -ERESTARTNOHAND;
                /*
                 * Report back to the user the time still remaining.
                 */
                if (rmtp && copy_to_user(rmtp, &it.it_value, sizeof *rmtp))
                        return -EFAULT;

                restart_block->fn = posix_cpu_nsleep_restart;
                restart_block->nanosleep.clockid = which_clock;
                restart_block->nanosleep.rmtp = rmtp;
                restart_block->nanosleep.expires = timespec_to_ns(rqtp);
        }
        return error;
}

static long posix_cpu_nsleep_restart(struct restart_block *restart_block)
{
        clockid_t which_clock = restart_block->nanosleep.clockid;
        struct timespec t;
        struct itimerspec it;
        int error;

        t = ns_to_timespec(restart_block->nanosleep.expires);

        error = do_cpu_nanosleep(which_clock, TIMER_ABSTIME, &t, &it);

        if (error == -ERESTART_RESTARTBLOCK) {
                struct timespec __user *rmtp = restart_block->nanosleep.rmtp;
                /*
                 * Report back to the user the time still remaining.
                 */
                if (rmtp && copy_to_user(rmtp, &it.it_value, sizeof *rmtp))
                        return -EFAULT;

                restart_block->nanosleep.expires = timespec_to_ns(&t);
        }
        return error;

}

#define PROCESS_CLOCK   MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_SCHED)
#define THREAD_CLOCK    MAKE_THREAD_CPUCLOCK(0, CPUCLOCK_SCHED)

static int process_cpu_clock_getres(const clockid_t which_clock,
                                    struct timespec *tp)
{
        return posix_cpu_clock_getres(PROCESS_CLOCK, tp);
}
static int process_cpu_clock_get(const clockid_t which_clock,
                                 struct timespec *tp)
{
        return posix_cpu_clock_get(PROCESS_CLOCK, tp);
}
static int process_cpu_timer_create(struct k_itimer *timer)
{
        timer->it_clock = PROCESS_CLOCK;
        return posix_cpu_timer_create(timer);
}
static int process_cpu_nsleep(const clockid_t which_clock, int flags,
                              struct timespec *rqtp,
                              struct timespec __user *rmtp)
{
        return posix_cpu_nsleep(PROCESS_CLOCK, flags, rqtp, rmtp);
}
static long process_cpu_nsleep_restart(struct restart_block *restart_block)
{
        return -EINVAL;
}
static int thread_cpu_clock_getres(const clockid_t which_clock,
                                   struct timespec *tp)
{
        return posix_cpu_clock_getres(THREAD_CLOCK, tp);
}
static int thread_cpu_clock_get(const clockid_t which_clock,
                                struct timespec *tp)
{
        return posix_cpu_clock_get(THREAD_CLOCK, tp);
}
static int thread_cpu_timer_create(struct k_itimer *timer)
{
        timer->it_clock = THREAD_CLOCK;
        return posix_cpu_timer_create(timer);
}

struct k_clock clock_posix_cpu = {
        .clock_getres   = posix_cpu_clock_getres,
        .clock_set      = posix_cpu_clock_set,
        .clock_get      = posix_cpu_clock_get,
        .timer_create   = posix_cpu_timer_create,
        .nsleep         = posix_cpu_nsleep,
        .nsleep_restart = posix_cpu_nsleep_restart,
        .timer_set      = posix_cpu_timer_set,
        .timer_del      = posix_cpu_timer_del,
        .timer_get      = posix_cpu_timer_get,
};

static __init int init_posix_cpu_timers(void)
{
        struct k_clock process = {
                .clock_getres   = process_cpu_clock_getres,
                .clock_get      = process_cpu_clock_get,
                .timer_create   = process_cpu_timer_create,
                .nsleep         = process_cpu_nsleep,
                .nsleep_restart = process_cpu_nsleep_restart,
        };
        struct k_clock thread = {
                .clock_getres   = thread_cpu_clock_getres,
                .clock_get      = thread_cpu_clock_get,
                .timer_create   = thread_cpu_timer_create,
        };
        struct timespec ts;

        posix_timers_register_clock(CLOCK_PROCESS_CPUTIME_ID, &process);
        posix_timers_register_clock(CLOCK_THREAD_CPUTIME_ID, &thread);

        cputime_to_timespec(cputime_one_jiffy, &ts);
        onecputick = ts.tv_nsec;
        WARN_ON(ts.tv_sec != 0);

        return 0;
}
__initcall(init_posix_cpu_timers);