runtime/base/mutex.cc

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "mutex.h"

#include <errno.h>
#include <sys/time.h>

#include "base/logging.h"
#include "cutils/atomic.h"
#include "cutils/atomic-inline.h"
#include "mutex-inl.h"
#include "runtime.h"
#include "scoped_thread_state_change.h"
#include "thread-inl.h"
#include "utils.h"

namespace art {

// This works on Mac OS 10.6 but hasn't been tested on older releases.
struct __attribute__((__may_alias__)) darwin_pthread_mutex_t {
  long padding0;
  int padding1;
  uint32_t padding2;
  int16_t padding3;
  int16_t padding4;
  uint32_t padding5;
  pthread_t darwin_pthread_mutex_owner;
  // ...other stuff we don't care about.
};

struct __attribute__((__may_alias__)) darwin_pthread_rwlock_t {
  long padding0;
  pthread_mutex_t padding1;
  int padding2;
  pthread_cond_t padding3;
  pthread_cond_t padding4;
  int padding5;
  int padding6;
  pthread_t darwin_pthread_rwlock_owner;
  // ...other stuff we don't care about.
};

struct __attribute__((__may_alias__)) glibc_pthread_mutex_t {
  int32_t padding0[2];
  int owner;
  // ...other stuff we don't care about.
};

struct __attribute__((__may_alias__)) glibc_pthread_rwlock_t {
#ifdef __LP64__
  int32_t padding0[6];
#else
  int32_t padding0[7];
#endif
  int writer;
  // ...other stuff we don't care about.
};

#if ART_USE_FUTEXES
static bool ComputeRelativeTimeSpec(timespec* result_ts, const timespec& lhs, const timespec& rhs) {
  const long int one_sec = 1000 * 1000 * 1000;  // one second in nanoseconds.
  result_ts->tv_sec = lhs.tv_sec - rhs.tv_sec;
  result_ts->tv_nsec = lhs.tv_nsec - rhs.tv_nsec;
  if (result_ts->tv_nsec < 0) {
    result_ts->tv_sec--;
    result_ts->tv_nsec += one_sec;
  } else if (result_ts->tv_nsec > one_sec) {
    result_ts->tv_sec++;
    result_ts->tv_nsec -= one_sec;
  }
  return result_ts->tv_sec < 0;
}
#endif

#if CONTENTION_LOGGING
// A guard for all_mutexes_ that's not a mutex (Mutexes must CAS to acquire and busy wait).
static AtomicInteger all_mutexes_guard_;
// All created mutexes guarded by all_mutexes_guard_.
std::set<BaseMutex*>* all_mutexes_;

class ScopedAllMutexesLock {
 public:
  explicit ScopedAllMutexesLock(const BaseMutex* mutex) : mutex_(mutex) {
    while (!all_mutexes_guard_.CompareAndSwap(0, reinterpret_cast<int32_t>(mutex))) {
      NanoSleep(100);
    }
  }
  ~ScopedAllMutexesLock() {
    while (!all_mutexes_guard_.CompareAndSwap(reinterpret_cast<int32_t>(mutex_), 0)) {
      NanoSleep(100);
    }
  }
 private:
  const BaseMutex* const mutex_;
};
#endif

BaseMutex::BaseMutex(const char* name, LockLevel level) : level_(level), name_(name) {
#if CONTENTION_LOGGING
  ScopedAllMutexesLock mu(this);
  if (all_mutexes_ == NULL) {
    // We leak the global set of all mutexes to avoid ordering issues in global variable
    // construction/destruction.
    all_mutexes_ = new std::set<BaseMutex*>();
  }
  all_mutexes_->insert(this);
#endif
}

BaseMutex::~BaseMutex() {
#if CONTENTION_LOGGING
  ScopedAllMutexesLock mu(this);
  all_mutexes_->erase(this);
#endif
}

void BaseMutex::DumpAll(std::ostream& os) {
#if CONTENTION_LOGGING
  os << "Mutex logging:\n";
  ScopedAllMutexesLock mu(reinterpret_cast<const BaseMutex*>(-1));
  typedef std::set<BaseMutex*>::const_iterator It;
  for (It it = all_mutexes_->begin(); it != all_mutexes_->end(); ++it) {
    BaseMutex* mutex = *it;
    mutex->Dump(os);
    os << "\n";
  }
#endif
}

void BaseMutex::CheckSafeToWait(Thread* self) {
  if (self == NULL) {
    CheckUnattachedThread(level_);
    return;
  }
  if (kDebugLocking) {
    CHECK(self->GetHeldMutex(level_) == this) << "Waiting on unacquired mutex: " << name_;
    bool bad_mutexes_held = false;
    for (int i = kLockLevelCount - 1; i >= 0; --i) {
      if (i != level_) {
        BaseMutex* held_mutex = self->GetHeldMutex(static_cast<LockLevel>(i));
        if (held_mutex != NULL) {
          LOG(ERROR) << "Holding \"" << held_mutex->name_ << "\" "
                     << "(level " << LockLevel(i) << ") while performing wait on "
                     << "\"" << name_ << "\" (level " << level_ << ")";
          bad_mutexes_held = true;
        }
      }
    }
    CHECK(!bad_mutexes_held);
  }
}

void BaseMutex::RecordContention(uint64_t blocked_tid, uint64_t owner_tid, uint64_t milli_time_blocked) {
#if CONTENTION_LOGGING
  ++contention_count_;
  wait_time_ += static_cast<uint32_t>(milli_time_blocked);  // May overflow.
  // This code is intentionally racy as it is only used for diagnostics.
  uint32_t slot = cur_content_log_entry_;
  if (contention_log_[slot].blocked_tid == blocked_tid &&
      contention_log_[slot].owner_tid == blocked_tid) {
    ++contention_log_[slot].count;
  } else {
    uint32_t new_slot;
    do {
      slot = cur_content_log_entry_;
      new_slot = (slot + 1) % kContentionLogSize;
    } while (!cur_content_log_entry_.CompareAndSwap(slot, new_slot));
    contention_log_[new_slot].blocked_tid = blocked_tid;
    contention_log_[new_slot].owner_tid = owner_tid;
    contention_log_[new_slot].count = 1;
  }
#endif
}

void BaseMutex::DumpContention(std::ostream& os) const {
#if CONTENTION_LOGGING
  uint32_t wait_time = wait_time_;
  uint32_t contention_count = contention_count_;
  if (contention_count == 0) {
    os << "never contended";
  } else {
    os << "contended " << contention_count << " times, average wait of contender " << (wait_time / contention_count) << "ms";
    SafeMap<uint64_t, size_t> most_common_blocker;
    SafeMap<uint64_t, size_t> most_common_blocked;
    typedef SafeMap<uint64_t, size_t>::const_iterator It;
    for (size_t i = 0; i < kContentionLogSize; ++i) {
      uint64_t blocked_tid = contention_log_[i].blocked_tid;
      uint64_t owner_tid = contention_log_[i].owner_tid;
      uint32_t count = contention_log_[i].count;
      if (count > 0) {
        It it = most_common_blocked.find(blocked_tid);
        if (it != most_common_blocked.end()) {
          most_common_blocked.Overwrite(blocked_tid, it->second + count);
        } else {
          most_common_blocked.Put(blocked_tid, count);
        }
        it = most_common_blocker.find(owner_tid);
        if (it != most_common_blocker.end()) {
          most_common_blocker.Overwrite(owner_tid, it->second + count);
        } else {
          most_common_blocker.Put(owner_tid, count);
        }
      }
    }
    uint64_t max_tid = 0;
    size_t max_tid_count = 0;
    for (It it = most_common_blocked.begin(); it != most_common_blocked.end(); ++it) {
      if (it->second > max_tid_count) {
        max_tid = it->first;
        max_tid_count = it->second;
      }
    }
    if (max_tid != 0) {
      os << " sample shows most blocked tid=" << max_tid;
    }
    max_tid = 0;
    max_tid_count = 0;
    for (It it = most_common_blocker.begin(); it != most_common_blocker.end(); ++it) {
      if (it->second > max_tid_count) {
        max_tid = it->first;
        max_tid_count = it->second;
      }
    }
    if (max_tid != 0) {
      os << " sample shows tid=" << max_tid << " owning during this time";
    }
  }
#endif
}


Mutex::Mutex(const char* name, LockLevel level, bool recursive)
    : BaseMutex(name, level), recursive_(recursive), recursion_count_(0) {
#if ART_USE_FUTEXES
  state_ = 0;
  exclusive_owner_ = 0;
  num_contenders_ = 0;
#elif defined(__BIONIC__) || defined(__APPLE__)
  // Use recursive mutexes for bionic and Apple otherwise the
  // non-recursive mutexes don't have TIDs to check lock ownership of.
  pthread_mutexattr_t attributes;
  CHECK_MUTEX_CALL(pthread_mutexattr_init, (&attributes));
  CHECK_MUTEX_CALL(pthread_mutexattr_settype, (&attributes, PTHREAD_MUTEX_RECURSIVE));
  CHECK_MUTEX_CALL(pthread_mutex_init, (&mutex_, &attributes));
  CHECK_MUTEX_CALL(pthread_mutexattr_destroy, (&attributes));
#else
  CHECK_MUTEX_CALL(pthread_mutex_init, (&mutex_, NULL));
#endif
}

Mutex::~Mutex() {
#if ART_USE_FUTEXES
  if (state_ != 0) {
    MutexLock mu(Thread::Current(), *Locks::runtime_shutdown_lock_);
    Runtime* runtime = Runtime::Current();
    bool shutting_down = (runtime == NULL) || runtime->IsShuttingDown();
    LOG(shutting_down ? WARNING : FATAL) << "destroying mutex with owner: " << exclusive_owner_;
  } else {
    CHECK_EQ(exclusive_owner_, 0U)  << "unexpectedly found an owner on unlocked mutex " << name_;
    CHECK_EQ(num_contenders_, 0) << "unexpectedly found a contender on mutex " << name_;
  }
#else
  // We can't use CHECK_MUTEX_CALL here because on shutdown a suspended daemon thread
  // may still be using locks.
  int rc = pthread_mutex_destroy(&mutex_);
  if (rc != 0) {
    errno = rc;
    // TODO: should we just not log at all if shutting down? this could be the logging mutex!
    MutexLock mu(Thread::Current(), *Locks::runtime_shutdown_lock_);
    Runtime* runtime = Runtime::Current();
    bool shutting_down = (runtime == NULL) || runtime->IsShuttingDown();
    PLOG(shutting_down ? WARNING : FATAL) << "pthread_mutex_destroy failed for " << name_;
  }
#endif
}

void Mutex::ExclusiveLock(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  if (kDebugLocking && !recursive_) {
    AssertNotHeld(self);
  }
  if (!recursive_ || !IsExclusiveHeld(self)) {
#if ART_USE_FUTEXES
    bool done = false;
    do {
      int32_t cur_state = state_;
      if (cur_state == 0) {
        // Change state from 0 to 1.
        done = android_atomic_acquire_cas(0, 1, &state_) == 0;
      } else {
        // Failed to acquire, hang up.
        ScopedContentionRecorder scr(this, GetExclusiveOwnerTid(), SafeGetTid(self));
        android_atomic_inc(&num_contenders_);
        if (futex(&state_, FUTEX_WAIT, 1, NULL, NULL, 0) != 0) {
          if (errno != EAGAIN) {
            PLOG(FATAL) << "futex wait failed for " << name_;
          }
        }
        android_atomic_dec(&num_contenders_);
      }
    } while (!done);
    DCHECK_EQ(state_, 1);
    exclusive_owner_ = SafeGetTid(self);
#else
    CHECK_MUTEX_CALL(pthread_mutex_lock, (&mutex_));
#endif
    RegisterAsLocked(self);
  }
  recursion_count_++;
  if (kDebugLocking) {
    CHECK(recursion_count_ == 1 || recursive_) << "Unexpected recursion count on mutex: "
        << name_ << " " << recursion_count_;
    AssertHeld(self);
  }
}

bool Mutex::ExclusiveTryLock(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  if (kDebugLocking && !recursive_) {
    AssertNotHeld(self);
  }
  if (!recursive_ || !IsExclusiveHeld(self)) {
#if ART_USE_FUTEXES
    bool done = false;
    do {
      int32_t cur_state = state_;
      if (cur_state == 0) {
        // Change state from 0 to 1.
        done = android_atomic_acquire_cas(0, 1, &state_) == 0;
      } else {
        return false;
      }
    } while (!done);
    DCHECK_EQ(state_, 1);
    exclusive_owner_ = SafeGetTid(self);
#else
    int result = pthread_mutex_trylock(&mutex_);
    if (result == EBUSY) {
      return false;
    }
    if (result != 0) {
      errno = result;
      PLOG(FATAL) << "pthread_mutex_trylock failed for " << name_;
    }
#endif
    RegisterAsLocked(self);
  }
  recursion_count_++;
  if (kDebugLocking) {
    CHECK(recursion_count_ == 1 || recursive_) << "Unexpected recursion count on mutex: "
        << name_ << " " << recursion_count_;
    AssertHeld(self);
  }
  return true;
}

void Mutex::ExclusiveUnlock(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  AssertHeld(self);
  recursion_count_--;
  if (!recursive_ || recursion_count_ == 0) {
    if (kDebugLocking) {
      CHECK(recursion_count_ == 0 || recursive_) << "Unexpected recursion count on mutex: "
          << name_ << " " << recursion_count_;
    }
    RegisterAsUnlocked(self);
#if ART_USE_FUTEXES
  bool done = false;
  do {
    int32_t cur_state = state_;
    if (cur_state == 1) {
      // We're no longer the owner.
      exclusive_owner_ = 0;
      // Change state to 0.
      done = android_atomic_release_cas(cur_state, 0, &state_) == 0;
      if (done) { // Spurious fail?
        // Wake a contender
        if (num_contenders_ > 0) {
          futex(&state_, FUTEX_WAKE, 1, NULL, NULL, 0);
        }
      }
    } else {
      // Logging acquires the logging lock, avoid infinite recursion in that case.
      if (this != Locks::logging_lock_) {
        LOG(FATAL) << "Unexpected state_ in unlock " << cur_state << " for " << name_;
      } else {
        LogMessageData data(__FILE__, __LINE__, INTERNAL_FATAL, -1);
        LogMessage::LogLine(data, StringPrintf("Unexpected state_ %d in unlock for %s",
                                               cur_state, name_).c_str());
        _exit(1);
      }
    }
  } while (!done);
#else
    CHECK_MUTEX_CALL(pthread_mutex_unlock, (&mutex_));
#endif
  }
}

bool Mutex::IsExclusiveHeld(const Thread* self) const {
  DCHECK(self == NULL || self == Thread::Current());
  bool result = (GetExclusiveOwnerTid() == SafeGetTid(self));
  if (kDebugLocking) {
    // Sanity debug check that if we think it is locked we have it in our held mutexes.
    if (result && self != NULL && level_ != kMonitorLock && !gAborting) {
      CHECK_EQ(self->GetHeldMutex(level_), this);
    }
  }
  return result;
}

uint64_t Mutex::GetExclusiveOwnerTid() const {
#if ART_USE_FUTEXES
  return exclusive_owner_;
#elif defined(__BIONIC__)
  return static_cast<uint64_t>((mutex_.value >> 16) & 0xffff);
#elif defined(__GLIBC__)
  return reinterpret_cast<const glibc_pthread_mutex_t*>(&mutex_)->owner;
#elif defined(__APPLE__)
  const darwin_pthread_mutex_t* dpmutex = reinterpret_cast<const darwin_pthread_mutex_t*>(&mutex_);
  pthread_t owner = dpmutex->darwin_pthread_mutex_owner;
  // 0 for unowned, -1 for PTHREAD_MTX_TID_SWITCHING
  // TODO: should we make darwin_pthread_mutex_owner volatile and recheck until not -1?
  if ((owner == (pthread_t)0) || (owner == (pthread_t)-1)) {
    return 0;
  }
  uint64_t tid;
  CHECK_PTHREAD_CALL(pthread_threadid_np, (owner, &tid), __FUNCTION__);  // Requires Mac OS 10.6
  return tid;
#else
#error unsupported C library
#endif
}

void Mutex::Dump(std::ostream& os) const {
  os << (recursive_ ? "recursive " : "non-recursive ")
      << name_
      << " level=" << static_cast<int>(level_)
      << " rec=" << recursion_count_
      << " owner=" << GetExclusiveOwnerTid() << " ";
  DumpContention(os);
}

std::ostream& operator<<(std::ostream& os, const Mutex& mu) {
  mu.Dump(os);
  return os;
}

ReaderWriterMutex::ReaderWriterMutex(const char* name, LockLevel level) :
    BaseMutex(name, level)
#if ART_USE_FUTEXES
    , state_(0), exclusive_owner_(0), num_pending_readers_(0), num_pending_writers_(0)
#endif
{ // NOLINT(whitespace/braces)
#if !ART_USE_FUTEXES
  CHECK_MUTEX_CALL(pthread_rwlock_init, (&rwlock_, NULL));
#endif
}

ReaderWriterMutex::~ReaderWriterMutex() {
#if ART_USE_FUTEXES
  CHECK_EQ(state_, 0);
  CHECK_EQ(exclusive_owner_, 0U);
  CHECK_EQ(num_pending_readers_, 0);
  CHECK_EQ(num_pending_writers_, 0);
#else
  // We can't use CHECK_MUTEX_CALL here because on shutdown a suspended daemon thread
  // may still be using locks.
  int rc = pthread_rwlock_destroy(&rwlock_);
  if (rc != 0) {
    errno = rc;
    // TODO: should we just not log at all if shutting down? this could be the logging mutex!
    MutexLock mu(Thread::Current(), *Locks::runtime_shutdown_lock_);
    Runtime* runtime = Runtime::Current();
    bool shutting_down = runtime == NULL || runtime->IsShuttingDown();
    PLOG(shutting_down ? WARNING : FATAL) << "pthread_rwlock_destroy failed for " << name_;
  }
#endif
}

void ReaderWriterMutex::ExclusiveLock(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  AssertNotExclusiveHeld(self);
#if ART_USE_FUTEXES
  bool done = false;
  do {
    int32_t cur_state = state_;
    if (cur_state == 0) {
      // Change state from 0 to -1.
      done = android_atomic_acquire_cas(0, -1, &state_) == 0;
    } else {
      // Failed to acquire, hang up.
      ScopedContentionRecorder scr(this, GetExclusiveOwnerTid(), SafeGetTid(self));
      android_atomic_inc(&num_pending_writers_);
      if (futex(&state_, FUTEX_WAIT, cur_state, NULL, NULL, 0) != 0) {
        if (errno != EAGAIN) {
          PLOG(FATAL) << "futex wait failed for " << name_;
        }
      }
      android_atomic_dec(&num_pending_writers_);
    }
  } while (!done);
  DCHECK_EQ(state_, -1);
  exclusive_owner_ = SafeGetTid(self);
#else
  CHECK_MUTEX_CALL(pthread_rwlock_wrlock, (&rwlock_));
#endif
  RegisterAsLocked(self);
  AssertExclusiveHeld(self);
}

void ReaderWriterMutex::ExclusiveUnlock(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  AssertExclusiveHeld(self);
  RegisterAsUnlocked(self);
#if ART_USE_FUTEXES
  bool done = false;
  do {
    int32_t cur_state = state_;
    if (cur_state == -1) {
      // We're no longer the owner.
      exclusive_owner_ = 0;
      // Change state from -1 to 0.
      done = android_atomic_release_cas(-1, 0, &state_) == 0;
      if (done) { // cmpxchg may fail due to noise?
        // Wake any waiters.
        if (num_pending_readers_ > 0 || num_pending_writers_ > 0) {
          futex(&state_, FUTEX_WAKE, -1, NULL, NULL, 0);
        }
      }
    } else {
      LOG(FATAL) << "Unexpected state_:" << cur_state << " for " << name_;
    }
  } while (!done);
#else
  CHECK_MUTEX_CALL(pthread_rwlock_unlock, (&rwlock_));
#endif
}

#if HAVE_TIMED_RWLOCK
bool ReaderWriterMutex::ExclusiveLockWithTimeout(Thread* self, int64_t ms, int32_t ns) {
  DCHECK(self == NULL || self == Thread::Current());
#if ART_USE_FUTEXES
  bool done = false;
  timespec end_abs_ts;
  InitTimeSpec(true, CLOCK_REALTIME, ms, ns, &end_abs_ts);
  do {
    int32_t cur_state = state_;
    if (cur_state == 0) {
      // Change state from 0 to -1.
      done = android_atomic_acquire_cas(0, -1, &state_) == 0;
    } else {
      // Failed to acquire, hang up.
      timespec now_abs_ts;
      InitTimeSpec(true, CLOCK_REALTIME, 0, 0, &now_abs_ts);
      timespec rel_ts;
      if (ComputeRelativeTimeSpec(&rel_ts, end_abs_ts, now_abs_ts)) {
        return false;  // Timed out.
      }
      ScopedContentionRecorder scr(this, GetExclusiveOwnerTid(), SafeGetTid(self));
      android_atomic_inc(&num_pending_writers_);
      if (futex(&state_, FUTEX_WAIT, cur_state, &rel_ts, NULL, 0) != 0) {
        if (errno == ETIMEDOUT) {
          android_atomic_dec(&num_pending_writers_);
          return false;  // Timed out.
        } else if (errno != EAGAIN && errno != EINTR) {
          PLOG(FATAL) << "timed futex wait failed for " << name_;
        }
      }
      android_atomic_dec(&num_pending_writers_);
    }
  } while (!done);
  exclusive_owner_ = SafeGetTid(self);
#else
  timespec ts;
  InitTimeSpec(true, CLOCK_REALTIME, ms, ns, &ts);
  int result = pthread_rwlock_timedwrlock(&rwlock_, &ts);
  if (result == ETIMEDOUT) {
    return false;
  }
  if (result != 0) {
    errno = result;
    PLOG(FATAL) << "pthread_rwlock_timedwrlock failed for " << name_;
  }
#endif
  RegisterAsLocked(self);
  AssertSharedHeld(self);
  return true;
}
#endif

bool ReaderWriterMutex::SharedTryLock(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
#if ART_USE_FUTEXES
  bool done = false;
  do {
    int32_t cur_state = state_;
    if (cur_state >= 0) {
      // Add as an extra reader.
      done = android_atomic_acquire_cas(cur_state, cur_state + 1, &state_) == 0;
    } else {
      // Owner holds it exclusively.
      return false;
    }
  } while (!done);
#else
  int result = pthread_rwlock_tryrdlock(&rwlock_);
  if (result == EBUSY) {
    return false;
  }
  if (result != 0) {
    errno = result;
    PLOG(FATAL) << "pthread_mutex_trylock failed for " << name_;
  }
#endif
  RegisterAsLocked(self);
  AssertSharedHeld(self);
  return true;
}

bool ReaderWriterMutex::IsExclusiveHeld(const Thread* self) const {
  DCHECK(self == NULL || self == Thread::Current());
  bool result = (GetExclusiveOwnerTid() == SafeGetTid(self));
  if (kDebugLocking) {
    // Sanity that if the pthread thinks we own the lock the Thread agrees.
    if (self != NULL && result)  {
      CHECK_EQ(self->GetHeldMutex(level_), this);
    }
  }
  return result;
}

bool ReaderWriterMutex::IsSharedHeld(const Thread* self) const {
  DCHECK(self == NULL || self == Thread::Current());
  bool result;
  if (UNLIKELY(self == NULL)) {  // Handle unattached threads.
    result = IsExclusiveHeld(self);  // TODO: a better best effort here.
  } else {
    result = (self->GetHeldMutex(level_) == this);
  }
  return result;
}

uint64_t ReaderWriterMutex::GetExclusiveOwnerTid() const {
#if ART_USE_FUTEXES
  int32_t state = state_;
  if (state == 0) {
    return 0;  // No owner.
  } else if (state > 0) {
    return -1;  // Shared.
  } else {
    return exclusive_owner_;
  }
#else
#if defined(__BIONIC__)
  return rwlock_.writerThreadId;
#elif defined(__GLIBC__)
  return reinterpret_cast<const glibc_pthread_rwlock_t*>(&rwlock_)->writer;
#elif defined(__APPLE__)
  const darwin_pthread_rwlock_t*
      dprwlock = reinterpret_cast<const darwin_pthread_rwlock_t*>(&rwlock_);
  pthread_t owner = dprwlock->darwin_pthread_rwlock_owner;
  if (owner == (pthread_t)0) {
    return 0;
  }
  uint64_t tid;
  CHECK_PTHREAD_CALL(pthread_threadid_np, (owner, &tid), __FUNCTION__);  // Requires Mac OS 10.6
  return tid;
#else
#error unsupported C library
#endif
#endif
}

void ReaderWriterMutex::Dump(std::ostream& os) const {
  os << name_
      << " level=" << static_cast<int>(level_)
      << " owner=" << GetExclusiveOwnerTid() << " ";
  DumpContention(os);
}

std::ostream& operator<<(std::ostream& os, const ReaderWriterMutex& mu) {
  mu.Dump(os);
  return os;
}

ConditionVariable::ConditionVariable(const char* name, Mutex& guard)
    : name_(name), guard_(guard) {
#if ART_USE_FUTEXES
  sequence_ = 0;
  num_waiters_ = 0;
#else
  CHECK_MUTEX_CALL(pthread_cond_init, (&cond_, NULL));
#endif
}

ConditionVariable::~ConditionVariable() {
#if ART_USE_FUTEXES
  if (num_waiters_!= 0) {
    MutexLock mu(Thread::Current(), *Locks::runtime_shutdown_lock_);
    Runtime* runtime = Runtime::Current();
    bool shutting_down = (runtime == NULL) || runtime->IsShuttingDown();
    LOG(shutting_down ? WARNING : FATAL) << "ConditionVariable::~ConditionVariable for " << name_
        << " called with " << num_waiters_ << " waiters.";
  }
#else
  // We can't use CHECK_MUTEX_CALL here because on shutdown a suspended daemon thread
  // may still be using condition variables.
  int rc = pthread_cond_destroy(&cond_);
  if (rc != 0) {
    errno = rc;
    MutexLock mu(Thread::Current(), *Locks::runtime_shutdown_lock_);
    Runtime* runtime = Runtime::Current();
    bool shutting_down = (runtime == NULL) || runtime->IsShuttingDown();
    PLOG(shutting_down ? WARNING : FATAL) << "pthread_cond_destroy failed for " << name_;
  }
#endif
}

void ConditionVariable::Broadcast(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  // TODO: enable below, there's a race in thread creation that causes false failures currently.
  // guard_.AssertExclusiveHeld(self);
  DCHECK_EQ(guard_.GetExclusiveOwnerTid(), SafeGetTid(self));
#if ART_USE_FUTEXES
  if (num_waiters_ > 0) {
    android_atomic_inc(&sequence_); // Indicate the broadcast occurred.
    bool done = false;
    do {
      int32_t cur_sequence = sequence_;
      // Requeue waiters onto mutex. The waiter holds the contender count on the mutex high ensuring
      // mutex unlocks will awaken the requeued waiter thread.
      done = futex(&sequence_, FUTEX_CMP_REQUEUE, 0,
                   reinterpret_cast<const timespec*>(std::numeric_limits<int32_t>::max()),
                   &guard_.state_, cur_sequence) != -1;
      if (!done) {
        if (errno != EAGAIN) {
          PLOG(FATAL) << "futex cmp requeue failed for " << name_;
        }
      }
    } while (!done);
  }
#else
  CHECK_MUTEX_CALL(pthread_cond_broadcast, (&cond_));
#endif
}

void ConditionVariable::Signal(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  guard_.AssertExclusiveHeld(self);
#if ART_USE_FUTEXES
  if (num_waiters_ > 0) {
    android_atomic_inc(&sequence_); // Indicate a signal occurred.
    // Futex wake 1 waiter who will then come and in contend on mutex. It'd be nice to requeue them
    // to avoid this, however, requeueing can only move all waiters.
    int num_woken = futex(&sequence_, FUTEX_WAKE, 1, NULL, NULL, 0);
    // Check something was woken or else we changed sequence_ before they had chance to wait.
    CHECK((num_woken == 0) || (num_woken == 1));
  }
#else
  CHECK_MUTEX_CALL(pthread_cond_signal, (&cond_));
#endif
}

void ConditionVariable::Wait(Thread* self) {
  guard_.CheckSafeToWait(self);
  WaitHoldingLocks(self);
}

void ConditionVariable::WaitHoldingLocks(Thread* self) {
  DCHECK(self == NULL || self == Thread::Current());
  guard_.AssertExclusiveHeld(self);
  unsigned int old_recursion_count = guard_.recursion_count_;
#if ART_USE_FUTEXES
  num_waiters_++;
  // Ensure the Mutex is contended so that requeued threads are awoken.
  android_atomic_inc(&guard_.num_contenders_);
  guard_.recursion_count_ = 1;
  int32_t cur_sequence = sequence_;
  guard_.ExclusiveUnlock(self);
  if (futex(&sequence_, FUTEX_WAIT, cur_sequence, NULL, NULL, 0) != 0) {
    // Futex failed, check it is an expected error.
    // EAGAIN == EWOULDBLK, so we let the caller try again.
    // EINTR implies a signal was sent to this thread.
    if ((errno != EINTR) && (errno != EAGAIN)) {
      PLOG(FATAL) << "futex wait failed for " << name_;
    }
  }
  guard_.ExclusiveLock(self);
  CHECK_GE(num_waiters_, 0);
  num_waiters_--;
  // We awoke and so no longer require awakes from the guard_'s unlock.
  CHECK_GE(guard_.num_contenders_, 0);
  android_atomic_dec(&guard_.num_contenders_);
#else
  guard_.recursion_count_ = 0;
  CHECK_MUTEX_CALL(pthread_cond_wait, (&cond_, &guard_.mutex_));
#endif
  guard_.recursion_count_ = old_recursion_count;
}

void ConditionVariable::TimedWait(Thread* self, int64_t ms, int32_t ns) {
  DCHECK(self == NULL || self == Thread::Current());
  guard_.AssertExclusiveHeld(self);
  guard_.CheckSafeToWait(self);
  unsigned int old_recursion_count = guard_.recursion_count_;
#if ART_USE_FUTEXES
  timespec rel_ts;
  InitTimeSpec(false, CLOCK_REALTIME, ms, ns, &rel_ts);
  num_waiters_++;
  // Ensure the Mutex is contended so that requeued threads are awoken.
  android_atomic_inc(&guard_.num_contenders_);
  guard_.recursion_count_ = 1;
  int32_t cur_sequence = sequence_;
  guard_.ExclusiveUnlock(self);
  if (futex(&sequence_, FUTEX_WAIT, cur_sequence, &rel_ts, NULL, 0) != 0) {
    if (errno == ETIMEDOUT) {
      // Timed out we're done.
    } else if ((errno == EINTR) || (errno == EAGAIN)) {
      // A signal or ConditionVariable::Signal/Broadcast has come in.
    } else {
      PLOG(FATAL) << "timed futex wait failed for " << name_;
    }
  }
  guard_.ExclusiveLock(self);
  CHECK_GE(num_waiters_, 0);
  num_waiters_--;
  // We awoke and so no longer require awakes from the guard_'s unlock.
  CHECK_GE(guard_.num_contenders_, 0);
  android_atomic_dec(&guard_.num_contenders_);
#else
#ifdef HAVE_TIMEDWAIT_MONOTONIC
#define TIMEDWAIT pthread_cond_timedwait_monotonic
  int clock = CLOCK_MONOTONIC;
#else
#define TIMEDWAIT pthread_cond_timedwait
  int clock = CLOCK_REALTIME;
#endif
  guard_.recursion_count_ = 0;
  timespec ts;
  InitTimeSpec(true, clock, ms, ns, &ts);
  int rc = TEMP_FAILURE_RETRY(TIMEDWAIT(&cond_, &guard_.mutex_, &ts));
  if (rc != 0 && rc != ETIMEDOUT) {
    errno = rc;
    PLOG(FATAL) << "TimedWait failed for " << name_;
  }
#endif
  guard_.recursion_count_ = old_recursion_count;
}

}  // namespace art