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 <sys/atomics.h>
#include <sys/mman.h>
#include <unistd.h>

#include "pthread_internal.h"

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

int pthread_condattr_init(pthread_condattr_t* attr) {
  if (attr == NULL) {
    return EINVAL;
  }
  *attr = PTHREAD_PROCESS_PRIVATE;
  return 0;
}

int pthread_condattr_getpshared(const pthread_condattr_t* attr, int* pshared) {
  if (attr == NULL || pshared == NULL) {
    return EINVAL;
  }
  *pshared = *attr;
  return 0;
}

int pthread_condattr_setpshared(pthread_condattr_t* attr, int pshared) {
  if (attr == NULL) {
    return EINVAL;
  }
  if (pshared != PTHREAD_PROCESS_SHARED && pshared != PTHREAD_PROCESS_PRIVATE) {
    return EINVAL;
  }
  *attr = pshared;
  return 0;
}

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

// We use one bit in condition variable values as the 'shared' flag
// The rest is a counter.
#define COND_SHARED_MASK        0x0001
#define COND_COUNTER_INCREMENT  0x0002
#define COND_COUNTER_MASK       (~COND_SHARED_MASK)

#define COND_IS_SHARED(c)  (((c)->value & COND_SHARED_MASK) != 0)

// 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.

int pthread_cond_init(pthread_cond_t* cond, const pthread_condattr_t* attr) {
  if (cond == NULL) {
    return EINVAL;
  }

  cond->value = 0;

  if (attr != NULL && *attr == PTHREAD_PROCESS_SHARED) {
    cond->value |= COND_SHARED_MASK;
  }

  return 0;
}

int pthread_cond_destroy(pthread_cond_t* cond) {
  if (cond == NULL) {
    return EINVAL;
  }

  cond->value = 0xdeadc04d;
  return 0;
}

// This function is used by pthread_cond_broadcast and
// pthread_cond_signal to atomically decrement the counter
// then wake up 'counter' threads.
static int __pthread_cond_pulse(pthread_cond_t* cond, int counter) {
  if (__predict_false(cond == NULL)) {
    return EINVAL;
  }

  long flags = (cond->value & ~COND_COUNTER_MASK);
  while (true) {
    long old_value = cond->value;
    long new_value = ((old_value - COND_COUNTER_INCREMENT) & COND_COUNTER_MASK) | flags;
    if (__bionic_cmpxchg(old_value, new_value, &cond->value) == 0) {
      break;
    }
  }

  // Ensure that all memory accesses previously made by this thread are
  // visible to the woken thread(s).  On the other side, the "wait"
  // code will issue any necessary barriers when locking the mutex.
  //
  // This may not strictly be necessary -- if the caller follows
  // recommended practice and holds the mutex before signaling the cond
  // var, the mutex ops will provide correct semantics.  If they don't
  // hold the mutex, they're subject to race conditions anyway.
  ANDROID_MEMBAR_FULL();

  __futex_wake_ex(&cond->value, COND_IS_SHARED(cond), counter);
  return 0;
}

__LIBC_HIDDEN__
int __pthread_cond_timedwait_relative(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* reltime) {
  int old_value = cond->value;

  pthread_mutex_unlock(mutex);
  int status = __futex_wait_ex(&cond->value, COND_IS_SHARED(cond), old_value, reltime);
  pthread_mutex_lock(mutex);

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

__LIBC_HIDDEN__
int __pthread_cond_timedwait(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* abstime, clockid_t clock) {
  timespec ts;
  timespec* tsp;

  if (abstime != NULL) {
    if (__timespec_to_absolute(&ts, abstime, clock) < 0) {
      return ETIMEDOUT;
    }
    tsp = &ts;
  } else {
    tsp = NULL;
  }

  return __pthread_cond_timedwait_relative(cond, mutex, tsp);
}

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

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

int pthread_cond_wait(pthread_cond_t* cond, pthread_mutex_t* mutex) {
  return __pthread_cond_timedwait(cond, mutex, NULL, CLOCK_REALTIME);
}

int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t * mutex, const timespec *abstime) {
  return __pthread_cond_timedwait(cond, mutex, abstime, CLOCK_REALTIME);
}

// TODO: this exists only for backward binary compatibility.
int pthread_cond_timedwait_monotonic(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* abstime) {
  return __pthread_cond_timedwait(cond, mutex, abstime, CLOCK_MONOTONIC);
}

int pthread_cond_timedwait_monotonic_np(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* abstime) {
  return __pthread_cond_timedwait(cond, mutex, abstime, CLOCK_MONOTONIC);
}

int pthread_cond_timedwait_relative_np(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* reltime) {
  return __pthread_cond_timedwait_relative(cond, mutex, reltime);
}

int pthread_cond_timeout_np(pthread_cond_t* cond, pthread_mutex_t* mutex, unsigned ms) {
  timespec ts;
  timespec_from_ms(ts, ms);
  return __pthread_cond_timedwait_relative(cond, mutex, &ts);
}