fusl/src/thread/pthread_barrier_wait.c

#include "pthread_impl.h"

static int pshared_barrier_wait(pthread_barrier_t* b) {
  int limit = (b->_b_limit & INT_MAX) + 1;
  int ret = 0;
  int v, w;

  if (limit == 1)
    return PTHREAD_BARRIER_SERIAL_THREAD;

  while ((v = a_cas(&b->_b_lock, 0, limit)))
    __wait(&b->_b_lock, &b->_b_waiters, v, 0);

  /* Wait for <limit> threads to get to the barrier */
  if (++b->_b_count == limit) {
    a_store(&b->_b_count, 0);
    ret = PTHREAD_BARRIER_SERIAL_THREAD;
    if (b->_b_waiters2)
      __wake(&b->_b_count, -1, 0);
  } else {
    a_store(&b->_b_lock, 0);
    if (b->_b_waiters)
      __wake(&b->_b_lock, 1, 0);
    while ((v = b->_b_count) > 0)
      __wait(&b->_b_count, &b->_b_waiters2, v, 0);
  }

  __vm_lock();

  /* Ensure all threads have a vm lock before proceeding */
  if (a_fetch_add(&b->_b_count, -1) == 1 - limit) {
    a_store(&b->_b_count, 0);
    if (b->_b_waiters2)
      __wake(&b->_b_count, -1, 0);
  } else {
    while ((v = b->_b_count))
      __wait(&b->_b_count, &b->_b_waiters2, v, 0);
  }

  /* Perform a recursive unlock suitable for self-sync'd destruction */
  do {
    v = b->_b_lock;
    w = b->_b_waiters;
  } while (a_cas(&b->_b_lock, v, v == INT_MIN + 1 ? 0 : v - 1) != v);

  /* Wake a thread waiting to reuse or destroy the barrier */
  if (v == INT_MIN + 1 || (v == 1 && w))
    __wake(&b->_b_lock, 1, 0);

  __vm_unlock();

  return ret;
}

struct instance {
  volatile int count;
  volatile int last;
  volatile int waiters;
  volatile int finished;
};

int pthread_barrier_wait(pthread_barrier_t* b) {
  int limit = b->_b_limit;
  struct instance* inst;

  /* Trivial case: count was set at 1 */
  if (!limit)
    return PTHREAD_BARRIER_SERIAL_THREAD;

  /* Process-shared barriers require a separate, inefficient wait */
  if (limit < 0)
    return pshared_barrier_wait(b);

  /* Otherwise we need a lock on the barrier object */
  while (a_swap(&b->_b_lock, 1))
    __wait(&b->_b_lock, &b->_b_waiters, 1, 1);
  inst = b->_b_inst;

  /* First thread to enter the barrier becomes the "instance owner" */
  if (!inst) {
    struct instance new_inst = {0};
    int spins = 200;
    b->_b_inst = inst = &new_inst;
    a_store(&b->_b_lock, 0);
    if (b->_b_waiters)
      __wake(&b->_b_lock, 1, 1);
    while (spins-- && !inst->finished)
      a_spin();
    a_inc(&inst->finished);
    while (inst->finished == 1)
      __syscall(SYS_futex, &inst->finished, FUTEX_WAIT | 128, 1, 0) !=
              -ENOSYS ||
          __syscall(SYS_futex, &inst->finished, FUTEX_WAIT, 1, 0);
    return PTHREAD_BARRIER_SERIAL_THREAD;
  }

  /* Last thread to enter the barrier wakes all non-instance-owners */
  if (++inst->count == limit) {
    b->_b_inst = 0;
    a_store(&b->_b_lock, 0);
    if (b->_b_waiters)
      __wake(&b->_b_lock, 1, 1);
    a_store(&inst->last, 1);
    if (inst->waiters)
      __wake(&inst->last, -1, 1);
  } else {
    a_store(&b->_b_lock, 0);
    if (b->_b_waiters)
      __wake(&b->_b_lock, 1, 1);
    __wait(&inst->last, &inst->waiters, 0, 1);
  }

  /* Last thread to exit the barrier wakes the instance owner */
  if (a_fetch_add(&inst->count, -1) == 1 && a_fetch_add(&inst->finished, 1))
    __wake(&inst->finished, 1, 1);

  return 0;
}