source: azure_iot_hub/trunk/musl-1.1.18/src/thread/pthread_cond_timedwait.c@ 390

#include "pthread_impl.h"

void __pthread_testcancel(void);
int __pthread_mutex_lock(pthread_mutex_t *);
int __pthread_mutex_unlock(pthread_mutex_t *);
int __pthread_setcancelstate(int, int *);

/*
 * struct waiter
 *
 * Waiter objects have automatic storage on the waiting thread, and
 * are used in building a linked list representing waiters currently
 * waiting on the condition variable or a group of waiters woken
 * together by a broadcast or signal; in the case of signal, this is a
 * degenerate list of one member.
 *
 * Waiter lists attached to the condition variable itself are
 * protected by the lock on the cv. Detached waiter lists are never
 * modified again, but can only be traversed in reverse order, and are
 * protected by the "barrier" locks in each node, which are unlocked
 * in turn to control wake order.
 *
 * Since process-shared cond var semantics do not necessarily allow
 * one thread to see another's automatic storage (they may be in
 * different processes), the waiter list is not used for the
 * process-shared case, but the structure is still used to store data
 * needed by the cancellation cleanup handler.
 */

struct waiter {
        struct waiter *prev, *next;
        volatile int state, barrier;
        volatile int *notify;
};

/* Self-synchronized-destruction-safe lock functions */

static inline void lock(volatile int *l)
{
        if (a_cas(l, 0, 1)) {
                a_cas(l, 1, 2);
                do __wait(l, 0, 2, 1);
                while (a_cas(l, 0, 2));
        }
}

static inline void unlock(volatile int *l)
{
        if (a_swap(l, 0)==2)
                __wake(l, 1, 1);
}

static inline void unlock_requeue(volatile int *l, volatile int *r, int w)
{
        a_store(l, 0);
        if (w) __wake(l, 1, 1);
        else __syscall(SYS_futex, l, FUTEX_REQUEUE|FUTEX_PRIVATE, 0, 1, r) != -ENOSYS
                || __syscall(SYS_futex, l, FUTEX_REQUEUE, 0, 1, r);
}

enum {
        WAITING,
        SIGNALED,
        LEAVING,
};

int __pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict m, const struct timespec *restrict ts)
{
        struct waiter node = { 0 };
        int e, seq, clock = c->_c_clock, cs, shared=0, oldstate, tmp;
        volatile int *fut;

        if ((m->_m_type&15) && (m->_m_lock&INT_MAX) != __pthread_self()->tid)
                return EPERM;

        if (ts && ts->tv_nsec >= 1000000000UL)
                return EINVAL;

        __pthread_testcancel();

        if (c->_c_shared) {
                shared = 1;
                fut = &c->_c_seq;
                seq = c->_c_seq;
                a_inc(&c->_c_waiters);
        } else {
                lock(&c->_c_lock);

                seq = node.barrier = 2;
                fut = &node.barrier;
                node.state = WAITING;
                node.next = c->_c_head;
                c->_c_head = &node;
                if (!c->_c_tail) c->_c_tail = &node;
                else node.next->prev = &node;

                unlock(&c->_c_lock);
        }

        __pthread_mutex_unlock(m);

        __pthread_setcancelstate(PTHREAD_CANCEL_MASKED, &cs);
        if (cs == PTHREAD_CANCEL_DISABLE) __pthread_setcancelstate(cs, 0);

        do e = __timedwait_cp(fut, seq, clock, ts, !shared);
        while (*fut==seq && (!e || e==EINTR));
        if (e == EINTR) e = 0;

        if (shared) {
                /* Suppress cancellation if a signal was potentially
                 * consumed; this is a legitimate form of spurious
                 * wake even if not. */
                if (e == ECANCELED && c->_c_seq != seq) e = 0;
                if (a_fetch_add(&c->_c_waiters, -1) == -0x7fffffff)
                        __wake(&c->_c_waiters, 1, 0);
                oldstate = WAITING;
                goto relock;
        }

        oldstate = a_cas(&node.state, WAITING, LEAVING);

        if (oldstate == WAITING) {
                /* Access to cv object is valid because this waiter was not
                 * yet signaled and a new signal/broadcast cannot return
                 * after seeing a LEAVING waiter without getting notified
                 * via the futex notify below. */

                lock(&c->_c_lock);
                
                if (c->_c_head == &node) c->_c_head = node.next;
                else if (node.prev) node.prev->next = node.next;
                if (c->_c_tail == &node) c->_c_tail = node.prev;
                else if (node.next) node.next->prev = node.prev;
                
                unlock(&c->_c_lock);

                if (node.notify) {
                        if (a_fetch_add(node.notify, -1)==1)
                                __wake(node.notify, 1, 1);
                }
        } else {
                /* Lock barrier first to control wake order. */
                lock(&node.barrier);
        }

relock:
        /* Errors locking the mutex override any existing error or
         * cancellation, since the caller must see them to know the
         * state of the mutex. */
        if ((tmp = pthread_mutex_lock(m))) e = tmp;

        if (oldstate == WAITING) goto done;

        if (!node.next) a_inc(&m->_m_waiters);

        /* Unlock the barrier that's holding back the next waiter, and
         * either wake it or requeue it to the mutex. */
        if (node.prev)
                unlock_requeue(&node.prev->barrier, &m->_m_lock, m->_m_type & 128);
        else
                a_dec(&m->_m_waiters);

        /* Since a signal was consumed, cancellation is not permitted. */
        if (e == ECANCELED) e = 0;

done:
        __pthread_setcancelstate(cs, 0);

        if (e == ECANCELED) {
                __pthread_testcancel();
                __pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, 0);
        }

        return e;
}

int __private_cond_signal(pthread_cond_t *c, int n)
{
        struct waiter *p, *first=0;
        volatile int ref = 0;
        int cur;

        lock(&c->_c_lock);
        for (p=c->_c_tail; n && p; p=p->prev) {
                if (a_cas(&p->state, WAITING, SIGNALED) != WAITING) {
                        ref++;
                        p->notify = &ref;
                } else {
                        n--;
                        if (!first) first=p;
                }
        }
        /* Split the list, leaving any remainder on the cv. */
        if (p) {
                if (p->next) p->next->prev = 0;
                p->next = 0;
        } else {
                c->_c_head = 0;
        }
        c->_c_tail = p;
        unlock(&c->_c_lock);

        /* Wait for any waiters in the LEAVING state to remove
         * themselves from the list before returning or allowing
         * signaled threads to proceed. */
        while ((cur = ref)) __wait(&ref, 0, cur, 1);

        /* Allow first signaled waiter, if any, to proceed. */
        if (first) unlock(&first->barrier);

        return 0;
}

weak_alias(__pthread_cond_timedwait, pthread_cond_timedwait);