/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2022 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/intrin/atomic.h"
#include "libc/intrin/futex.internal.h"
#include "libc/thread/thread.h"

static int pthread_rwlock_wrlock_spin(pthread_rwlock_t *rwlock, int expect,
                                      int tries) {
  if (tries < 7) {
    volatile int i;
    for (i = 0; i != 1 << tries; i++) {
    }
    tries++;
  } else {
    atomic_fetch_add(&rwlock->waits, 1);
    _futex_wait(&rwlock->lock, expect, rwlock->pshared, &(struct timespec){1});
    atomic_fetch_sub(&rwlock->waits, 1);
  }
  return tries;
}

/**
 * Acquires write lock on read-write lock.
 *
 * @return 0 on success, or errno on error
 */
int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock) {
  int old, tries;
  for (tries = 0;;) {
    if (!(old = atomic_load_explicit(&rwlock->lock, memory_order_relaxed))) {
      do {
        if (atomic_compare_exchange_weak_explicit(&rwlock->lock, &old, -1,
                                                  memory_order_acquire,
                                                  memory_order_relaxed)) {
          return 0;
        }
      } while (!old);
    }
    tries = pthread_rwlock_wrlock_spin(rwlock, old, tries);
  }
}