cosmopolitan/libc/intrin/pthread_mutex_lock.c

/*-*- 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/assert.h"
#include "libc/calls/calls.h"
#include "libc/dce.h"
#include "libc/errno.h"
#include "libc/intrin/asmflag.h"
#include "libc/intrin/atomic.h"
#include "libc/intrin/futex.internal.h"
#include "libc/intrin/pthread.h"
#include "libc/intrin/spinlock.h"
#include "libc/linux/futex.h"
#include "libc/nexgen32e/threaded.h"
#include "libc/sysv/consts/futex.h"
#include "libc/sysv/consts/nr.h"

static int pthread_mutex_lock_spin(pthread_mutex_t *mutex, int expect,
                                   int tries) {
  if (tries < 7) {
    volatile int i;
    for (i = 0; i != 1 << tries; i++) {
    }
    tries++;
  } else if (IsLinux() || IsOpenbsd()) {
    atomic_fetch_add(&mutex->waits, 1);
    _futex_wait(&mutex->lock, expect, mutex->pshared, &(struct timespec){1});
    atomic_fetch_sub(&mutex->waits, 1);
  } else {
    pthread_yield();
  }
  return tries;
}

/**
 * Locks mutex.
 *
 * Here's an example of using a normal mutex:
 *
 *     pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
 *     pthread_mutex_lock(&lock);
 *     // do work...
 *     pthread_mutex_unlock(&lock);
 *     pthread_mutex_destroy(&lock);
 *
 * Cosmopolitan permits succinct notation for normal mutexes:
 *
 *     pthread_mutex_t lock = {0};
 *     pthread_mutex_lock(&lock);
 *     // do work...
 *     pthread_mutex_unlock(&lock);
 *
 * Here's an example of the proper way to do recursive mutexes:
 *
 *     pthread_mutex_t lock;
 *     pthread_mutexattr_t attr;
 *     pthread_mutexattr_init(&attr);
 *     pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
 *     pthread_mutex_init(&lock, &attr);
 *     pthread_mutexattr_destroy(&attr);
 *     pthread_mutex_lock(&lock);
 *     // do work...
 *     pthread_mutex_unlock(&lock);
 *     pthread_mutex_destroy(&lock);
 *
 * Alternatively, Cosmopolitan lets you do the folllowing instead:
 *
 *     pthread_mutex_t lock = {PTHREAD_MUTEX_RECURSIVE};
 *     pthread_mutex_lock(&lock);
 *     // do work...
 *     pthread_mutex_unlock(&lock);
 *
 * Microbenchmarks for single-threaded lock + unlock:
 *
 *     pthread_spinlock_t          :    12c (  4ns)
 *     PTHREAD_MUTEX_NORMAL        :    37c ( 12ns)
 *     PTHREAD_MUTEX_RECURSIVE     :    22c (  7ns)
 *     PTHREAD_MUTEX_ERRORCHECK    :    27c (  9ns)
 *
 * Microbenchmarks for multi-threaded lock + unlock:
 *
 *     pthread_spinlock_t          : 2,396c (774ns)
 *     PTHREAD_MUTEX_NORMAL        :   535c (173ns)
 *     PTHREAD_MUTEX_RECURSIVE     : 1,045c (338ns)
 *     PTHREAD_MUTEX_ERRORCHECK    :   917c (296ns)
 *
 * @return 0 on success, or error number on failure
 * @see pthread_spin_lock()
 */
int pthread_mutex_lock(pthread_mutex_t *mutex) {
  int c, me, owner, tries;
  switch (mutex->type) {
    case PTHREAD_MUTEX_NORMAL:
      // From Futexes Are Tricky Version 1.1 § Mutex, Take 3;
      // Ulrich Drepper, Red Hat Incorporated, June 27, 2004.
      c = 0;
      if (!atomic_compare_exchange_strong_explicit(&mutex->lock, &c, 1,
                                                   memory_order_acquire,
                                                   memory_order_relaxed)) {
        if (c != 2) {
          c = atomic_exchange_explicit(&mutex->lock, 2, memory_order_acquire);
        }
        while (c) {
          _futex_wait(&mutex->lock, 2, mutex->pshared, 0);
          c = atomic_exchange_explicit(&mutex->lock, 2, memory_order_acquire);
        }
      }
      return 0;
    case PTHREAD_MUTEX_RECURSIVE:
    case PTHREAD_MUTEX_ERRORCHECK:
      for (tries = 0, me = gettid();;) {
        owner = atomic_load_explicit(&mutex->lock, memory_order_relaxed);
        if (!owner && atomic_compare_exchange_weak_explicit(
                          &mutex->lock, &owner, me, memory_order_acquire,
                          memory_order_relaxed)) {
          break;
        } else if (owner == me) {
          if (mutex->type != PTHREAD_MUTEX_ERRORCHECK) {
            break;
          } else {
            assert(!"deadlock");
            return EDEADLK;
          }
        }
        tries = pthread_mutex_lock_spin(mutex, owner, tries);
      }
      ++mutex->reent;
      return 0;
    default:
      assert(!"badlock");
      return EINVAL;
  }
}