libc/bionic/pthread_cond.cpp

/*
 * Copyright (C) 2008 The Android Open Source Project
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *  * Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *  * Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <pthread.h>

#include <errno.h>
#include <limits.h>
#include <stdatomic.h>
#include <sys/mman.h>
#include <time.h>
#include <unistd.h>

#include "pthread_internal.h"

#include "private/bionic_futex.h"
#include "private/bionic_time_conversions.h"
#include "private/bionic_tls.h"

// XXX *technically* there is a race condition that could allow
// XXX a signal to be missed.  If thread A is preempted in _wait()
// XXX after unlocking the mutex and before waiting, and if other
// XXX threads call signal or broadcast UINT_MAX/2 times (exactly),
// XXX before thread A is scheduled again and calls futex_wait(),
// XXX then the signal will be lost.

// We use one bit in pthread_condattr_t (long) values as the 'shared' flag
// and one bit for the clock type (CLOCK_REALTIME is ((clockid_t) 1), and
// CLOCK_MONOTONIC is ((clockid_t) 0).). The rest of the bits are a counter.
//
// The 'value' field pthread_cond_t has the same layout.

#define COND_SHARED_MASK 0x0001
#define COND_CLOCK_MASK 0x0002
#define COND_COUNTER_STEP 0x0004
#define COND_FLAGS_MASK (COND_SHARED_MASK | COND_CLOCK_MASK)
#define COND_COUNTER_MASK (~COND_FLAGS_MASK)

#define COND_IS_SHARED(c) (((c) & COND_SHARED_MASK) != 0)
#define COND_GET_CLOCK(c) (((c) & COND_CLOCK_MASK) >> 1)
#define COND_SET_CLOCK(attr, c) ((attr) | (c << 1))

int pthread_condattr_init(pthread_condattr_t* attr) {
  *attr = 0;
  *attr |= PTHREAD_PROCESS_PRIVATE;
  *attr |= (CLOCK_REALTIME << 1);
  return 0;
}

int pthread_condattr_getpshared(const pthread_condattr_t* attr, int* pshared) {
  *pshared = static_cast<int>(COND_IS_SHARED(*attr));
  return 0;
}

int pthread_condattr_setpshared(pthread_condattr_t* attr, int pshared) {
  if (pshared != PTHREAD_PROCESS_SHARED && pshared != PTHREAD_PROCESS_PRIVATE) {
    return EINVAL;
  }

  *attr |= pshared;
  return 0;
}

int pthread_condattr_getclock(const pthread_condattr_t* attr, clockid_t* clock) {
  *clock = COND_GET_CLOCK(*attr);
  return 0;
}

int pthread_condattr_setclock(pthread_condattr_t* attr, clockid_t clock) {
  if (clock != CLOCK_MONOTONIC && clock != CLOCK_REALTIME) {
    return EINVAL;
  }

  *attr = COND_SET_CLOCK(*attr, clock);
  return 0;
}

int pthread_condattr_destroy(pthread_condattr_t* attr) {
  *attr = 0xdeada11d;
  return 0;
}

struct pthread_cond_internal_t {
  atomic_uint state;

  bool process_shared() {
    return COND_IS_SHARED(atomic_load_explicit(&state, memory_order_relaxed));
  }

  int get_clock() {
    return COND_GET_CLOCK(atomic_load_explicit(&state, memory_order_relaxed));
  }

#if defined(__LP64__)
  char __reserved[44];
#endif
};

static_assert(sizeof(pthread_cond_t) == sizeof(pthread_cond_internal_t),
              "pthread_cond_t should actually be pthread_cond_internal_t in implementation.");

// For binary compatibility with old version of pthread_cond_t, we can't use more strict alignment
// than 4-byte alignment.
static_assert(alignof(pthread_cond_t) == 4,
              "pthread_cond_t should fulfill the alignment requirement of pthread_cond_internal_t.");

static pthread_cond_internal_t* __get_internal_cond(pthread_cond_t* cond_interface) {
  return reinterpret_cast<pthread_cond_internal_t*>(cond_interface);
}

int pthread_cond_init(pthread_cond_t* cond_interface, const pthread_condattr_t* attr) {
  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);

  unsigned int init_state = 0;
  if (attr != NULL) {
    init_state = (*attr & COND_FLAGS_MASK);
  }
  atomic_init(&cond->state, init_state);

  return 0;
}

int pthread_cond_destroy(pthread_cond_t* cond_interface) {
  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);
  atomic_store_explicit(&cond->state, 0xdeadc04d, memory_order_relaxed);
  return 0;
}

// This function is used by pthread_cond_broadcast and
// pthread_cond_signal to atomically decrement the counter
// then wake up thread_count threads.
static int __pthread_cond_pulse(pthread_cond_internal_t* cond, int thread_count) {
  // We don't use a release/seq_cst fence here. Because pthread_cond_wait/signal can't be
  // used as a method for memory synchronization by itself. It should always be used with
  // pthread mutexes. Note that Spurious wakeups from pthread_cond_wait/timedwait may occur,
  // so when using condition variables there is always a boolean predicate involving shared
  // variables associated with each condition wait that is true if the thread should proceed.
  // If the predicate is seen true before a condition wait, pthread_cond_wait/timedwait will
  // not be called. That's why pthread_wait/signal pair can't be used as a method for memory
  // synchronization. And it doesn't help even if we use any fence here.

  // The increase of value should leave flags alone, even if the value can overflows.
  atomic_fetch_add_explicit(&cond->state, COND_COUNTER_STEP, memory_order_relaxed);

  __futex_wake_ex(&cond->state, cond->process_shared(), thread_count);
  return 0;
}

static int __pthread_cond_timedwait_relative(pthread_cond_internal_t* cond, pthread_mutex_t* mutex,
                                             const timespec* rel_timeout_or_null) {
  unsigned int old_state = atomic_load_explicit(&cond->state, memory_order_relaxed);

  pthread_mutex_unlock(mutex);
  int status = __futex_wait_ex(&cond->state, cond->process_shared(), old_state, rel_timeout_or_null);
  pthread_mutex_lock(mutex);

  if (status == -ETIMEDOUT) {
    return ETIMEDOUT;
  }
  return 0;
}

static int __pthread_cond_timedwait(pthread_cond_internal_t* cond, pthread_mutex_t* mutex,
                                    const timespec* abs_timeout_or_null, clockid_t clock) {
  timespec ts;
  timespec* rel_timeout = NULL;

  if (abs_timeout_or_null != NULL) {
    rel_timeout = &ts;
    if (!timespec_from_absolute_timespec(*rel_timeout, *abs_timeout_or_null, clock)) {
      return ETIMEDOUT;
    }
  }

  return __pthread_cond_timedwait_relative(cond, mutex, rel_timeout);
}

int pthread_cond_broadcast(pthread_cond_t* cond_interface) {
  return __pthread_cond_pulse(__get_internal_cond(cond_interface), INT_MAX);
}

int pthread_cond_signal(pthread_cond_t* cond_interface) {
  return __pthread_cond_pulse(__get_internal_cond(cond_interface), 1);
}

int pthread_cond_wait(pthread_cond_t* cond_interface, pthread_mutex_t* mutex) {
  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);
  return __pthread_cond_timedwait(cond, mutex, NULL, cond->get_clock());
}

int pthread_cond_timedwait(pthread_cond_t *cond_interface, pthread_mutex_t * mutex,
                           const timespec *abstime) {

  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);
  return __pthread_cond_timedwait(cond, mutex, abstime, cond->get_clock());
}

#if !defined(__LP64__)
// TODO: this exists only for backward binary compatibility on 32 bit platforms.
extern "C" int pthread_cond_timedwait_monotonic(pthread_cond_t* cond_interface,
                                                pthread_mutex_t* mutex,
                                                const timespec* abs_timeout) {

  return __pthread_cond_timedwait(__get_internal_cond(cond_interface), mutex, abs_timeout,
                                  CLOCK_MONOTONIC);
}

extern "C" int pthread_cond_timedwait_monotonic_np(pthread_cond_t* cond_interface,
                                                   pthread_mutex_t* mutex,
                                                   const timespec* abs_timeout) {
  return pthread_cond_timedwait_monotonic(cond_interface, mutex, abs_timeout);
}

extern "C" int pthread_cond_timedwait_relative_np(pthread_cond_t* cond_interface,
                                                  pthread_mutex_t* mutex,
                                                  const timespec* rel_timeout) {

  return __pthread_cond_timedwait_relative(__get_internal_cond(cond_interface), mutex, rel_timeout);
}

extern "C" int pthread_cond_timeout_np(pthread_cond_t* cond_interface,
                                       pthread_mutex_t* mutex, unsigned ms) {
  timespec ts;
  timespec_from_ms(ts, ms);
  return pthread_cond_timedwait_relative_np(cond_interface, mutex, &ts);
}
#endif // !defined(__LP64__)