fusl/src/thread/pthread_cond_timedwait.c

#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 | 128, 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);