zircon/third_party/ulib/musl/pthread/pthread_barrier_wait.c

#include "futex_impl.h"
#include "threads_impl.h"

struct instance {
  atomic_uint count;
  atomic_int last;
  atomic_int waiters;
  atomic_int finished;
};

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

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

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

  /* First thread to enter the barrier becomes the "instance owner" */
  if (!inst) {
    struct instance new_inst = {
        ATOMIC_VAR_INIT(0),
        ATOMIC_VAR_INIT(0),
        ATOMIC_VAR_INIT(0),
        ATOMIC_VAR_INIT(0),
    };
    int spins = 200;
    b->_b_inst = inst = &new_inst;
    atomic_store(&b->_b_lock, 0);
    if (atomic_load(&b->_b_waiters))
      _zx_futex_wake(&b->_b_lock, 1);
    while (spins-- && !atomic_load(&inst->finished))
      a_spin();
    atomic_fetch_add(&inst->finished, 1);
    while (atomic_load(&inst->finished) == 1)
      _zx_futex_wait(&inst->finished, 1, ZX_HANDLE_INVALID, ZX_TIME_INFINITE);
    return PTHREAD_BARRIER_SERIAL_THREAD;
  }

  /* Last thread to enter the barrier wakes all
   * non-instance-owners. One is added to the count since we fetch
   * before adding, and another one to account for the fact that the
   * first thread (the "instance owner" above) never gets here. */
  if (atomic_fetch_add(&inst->count, 1) + 2 == limit) {
    b->_b_inst = 0;
    atomic_store(&b->_b_lock, 0);
    if (atomic_load(&b->_b_waiters))
      _zx_futex_wake(&b->_b_lock, 1);
    atomic_store(&inst->last, 1);
    if (inst->waiters)
      _zx_futex_wake(&inst->last, UINT32_MAX);
  } else {
    atomic_store(&b->_b_lock, 0);
    if (atomic_load(&b->_b_waiters))
      _zx_futex_wake(&b->_b_lock, 1);
    __wait(&inst->last, &inst->waiters, 0);
  }

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

  return 0;
}