UnlikelyLock.cc

/*
 * Copyright (c) 2008-2014, Pedigree Developers
 *
 * Please see the CONTRIB file in the root of the source tree for a full
 * list of contributors.
 *
 * Permission to use, copy, modify, and 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 "pedigree/kernel/LockGuard.h"
#include "pedigree/kernel/Log.h"
#include "pedigree/kernel/utilities/UnlikelyLock.h"
 
UnlikelyLock::UnlikelyLock()
    : m_Lock(), m_Condition(), m_nReaders(0), m_nWaitingWriters(0), m_bActiveWriter(false) {}
 
UnlikelyLock::~UnlikelyLock() {
  LockGuard<Mutex> guard(m_Lock);
  if (m_nReaders || m_nWaitingWriters || m_bActiveWriter) {
    FATAL("Destroying an owned or contended UnlikelyLock.");
  }
}
 
void UnlikelyLock::enter() {
  LockGuard<Mutex> guard(m_Lock);
  while (m_bActiveWriter || m_nWaitingWriters) {
    m_Condition.waitForCompletion(m_Lock);
  }
 
  ++m_nReaders;
}
 
void UnlikelyLock::leave() {
  LockGuard<Mutex> guard(m_Lock);
  if (!m_nReaders) {
    FATAL("UnlikelyLock::leave without a matching enter.");
  }
  if (!--m_nReaders) {
    // A signal can select a reader, which must remain gated while a
    // writer is pending. Wake the writer set as well.
    m_Condition.broadcast();
  }
}
 
void UnlikelyLock::acquire() {
  LockGuard<Mutex> guard(m_Lock);
  ++m_nWaitingWriters;
  while (m_bActiveWriter || m_nReaders) {
    m_Condition.waitForCompletion(m_Lock);
  }
 
  --m_nWaitingWriters;
  m_bActiveWriter = true;
}
 
void UnlikelyLock::release() {
  LockGuard<Mutex> guard(m_Lock);
  if (!m_bActiveWriter) {
    FATAL("UnlikelyLock::release without writer ownership.");
  }
  m_bActiveWriter = false;
  m_Condition.broadcast();
}