Remove XP Condition Variable Affordances

base/synchronization/condition_variable_win.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/synchronization/condition_variable.h"
 
-#include <windows.h>
-#include <stack>
-
-#include "base/compiler_specific.h"
-#include "base/macros.h"
 #include "base/synchronization/lock.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 
-namespace {
-// We can't use the linker supported delay-load for kernel32 so all this
-// cruft here is to manually late-bind the needed functions.
-typedef void (WINAPI *InitializeConditionVariableFn)(PCONDITION_VARIABLE);
-typedef BOOL (WINAPI *SleepConditionVariableCSFn)(PCONDITION_VARIABLE,
-                                                  PCRITICAL_SECTION, DWORD);
-typedef void (WINAPI *WakeConditionVariableFn)(PCONDITION_VARIABLE);
-typedef void (WINAPI *WakeAllConditionVariableFn)(PCONDITION_VARIABLE);
+namespace base {
 
-InitializeConditionVariableFn initialize_condition_variable_fn;
-SleepConditionVariableCSFn sleep_condition_variable_fn;
-WakeConditionVariableFn wake_condition_variable_fn;
-WakeAllConditionVariableFn wake_all_condition_variable_fn;
-
-bool BindVistaCondVarFunctions() {
-  HMODULE kernel32 = GetModuleHandleA("kernel32.dll");
-  initialize_condition_variable_fn =
-      reinterpret_cast<InitializeConditionVariableFn>(
-          GetProcAddress(kernel32, "InitializeConditionVariable"));
-  if (!initialize_condition_variable_fn)
-    return false;
-  sleep_condition_variable_fn =
-      reinterpret_cast<SleepConditionVariableCSFn>(
-          GetProcAddress(kernel32, "SleepConditionVariableCS"));
-  if (!sleep_condition_variable_fn)
-    return false;
-  wake_condition_variable_fn =
-      reinterpret_cast<WakeConditionVariableFn>(
-          GetProcAddress(kernel32, "WakeConditionVariable"));
-  if (!wake_condition_variable_fn)
-    return false;
-  wake_all_condition_variable_fn =
-      reinterpret_cast<WakeAllConditionVariableFn>(
-          GetProcAddress(kernel32, "WakeAllConditionVariable"));
-  if (!wake_all_condition_variable_fn)
-    return false;
-  return true;
+ConditionVariable::ConditionVariable(Lock* user_lock)
+    : crit_sec_(user_lock->lock_.native_handle())
+#if DCHECK_IS_ON()
+    , user_lock_(user_lock)
+#endif
+{
+  DCHECK(user_lock);
+  InitializeConditionVariable(&cv_);
 }
 
-}  // namespace.
+ConditionVariable::~ConditionVariable() = default;
 
-namespace base {
-// Abstract base class of the pimpl idiom.
-class ConditionVarImpl {
- public:
-  virtual ~ConditionVarImpl() {};
-  virtual void Wait() = 0;
-  virtual void TimedWait(const TimeDelta& max_time) = 0;
-  virtual void Broadcast() = 0;
-  virtual void Signal() = 0;
-};
-
-///////////////////////////////////////////////////////////////////////////////
-// Windows Vista and Win7 implementation.
-///////////////////////////////////////////////////////////////////////////////
-
-class WinVistaCondVar: public ConditionVarImpl {
- public:
-  WinVistaCondVar(Lock* user_lock);
-  ~WinVistaCondVar() override {}
-  // Overridden from ConditionVarImpl.
-  void Wait() override;
-  void TimedWait(const TimeDelta& max_time) override;
-  void Broadcast() override;
-  void Signal() override;
-
- private:
-  base::Lock& user_lock_;
-  CONDITION_VARIABLE cv_;
-};
-
-WinVistaCondVar::WinVistaCondVar(Lock* user_lock)
-    : user_lock_(*user_lock) {
-  initialize_condition_variable_fn(&cv_);
-  DCHECK(user_lock);
-}
-
-void WinVistaCondVar::Wait() {
+void ConditionVariable::Wait() {
   TimedWait(TimeDelta::FromMilliseconds(INFINITE));
 }
 
-void WinVistaCondVar::TimedWait(const TimeDelta& max_time) {
+void ConditionVariable::TimedWait(const TimeDelta& max_time) {
   base::ThreadRestrictions::AssertWaitAllowed();
   DWORD timeout = static_cast<DWORD>(max_time.InMilliseconds());
-  CRITICAL_SECTION* cs = user_lock_.lock_.native_handle();
 
 #if DCHECK_IS_ON()
-  user_lock_.CheckHeldAndUnmark();
+  user_lock_->CheckHeldAndUnmark();
 #endif
 
-  if (FALSE == sleep_condition_variable_fn(&cv_, cs, timeout)) {
+  if (FALSE == SleepConditionVariableCS(&cv_, crit_sec_, timeout)) {
     DCHECK(GetLastError() != WAIT_TIMEOUT);
   }
 
-#if !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)
-  user_lock_.CheckUnheldAndMark();
+#if DCHECK_IS_ON()
+  user_lock_->CheckUnheldAndMark();
 #endif
 }
 
-void WinVistaCondVar::Broadcast() {
-  wake_all_condition_variable_fn(&cv_);
-}
-
-void WinVistaCondVar::Signal() {
-  wake_condition_variable_fn(&cv_);
-}
-
-///////////////////////////////////////////////////////////////////////////////
-// Windows XP implementation.
-///////////////////////////////////////////////////////////////////////////////
-
-class WinXPCondVar : public ConditionVarImpl {
- public:
-  WinXPCondVar(Lock* user_lock);
-  ~WinXPCondVar() override;
-  // Overridden from ConditionVarImpl.
-  void Wait() override;
-  void TimedWait(const TimeDelta& max_time) override;
-  void Broadcast() override;
-  void Signal() override;
-
-  // Define Event class that is used to form circularly linked lists.
-  // The list container is an element with NULL as its handle_ value.
-  // The actual list elements have a non-zero handle_ value.
-  // All calls to methods MUST be done under protection of a lock so that links
-  // can be validated.  Without the lock, some links might asynchronously
-  // change, and the assertions would fail (as would list change operations).
-  class Event {
-   public:
-    // Default constructor with no arguments creates a list container.
-    Event();
-    ~Event();
-
-    // InitListElement transitions an instance from a container, to an element.
-    void InitListElement();
-
-    // Methods for use on lists.
-    bool IsEmpty() const;
-    void PushBack(Event* other);
-    Event* PopFront();
-    Event* PopBack();
-
-    // Methods for use on list elements.
-    // Accessor method.
-    HANDLE handle() const;
-    // Pull an element from a list (if it's in one).
-    Event* Extract();
-
-    // Method for use on a list element or on a list.
-    bool IsSingleton() const;
-
-   private:
-    // Provide pre/post conditions to validate correct manipulations.
-    bool ValidateAsDistinct(Event* other) const;
-    bool ValidateAsItem() const;
-    bool ValidateAsList() const;
-    bool ValidateLinks() const;
-
-    HANDLE handle_;
-    Event* next_;
-    Event* prev_;
-    DISALLOW_COPY_AND_ASSIGN(Event);
-  };
-
-  // Note that RUNNING is an unlikely number to have in RAM by accident.
-  // This helps with defensive destructor coding in the face of user error.
-  enum RunState { SHUTDOWN = 0, RUNNING = 64213 };
-
-  // Internal implementation methods supporting Wait().
-  Event* GetEventForWaiting();
-  void RecycleEvent(Event* used_event);
-
-  RunState run_state_;
-
-  // Private critical section for access to member data.
-  base::Lock internal_lock_;
-
-  // Lock that is acquired before calling Wait().
-  base::Lock& user_lock_;
-
-  // Events that threads are blocked on.
-  Event waiting_list_;
-
-  // Free list for old events.
-  Event recycling_list_;
-  int recycling_list_size_;
-
-  // The number of allocated, but not yet deleted events.
-  int allocation_counter_;
-};
-
-WinXPCondVar::WinXPCondVar(Lock* user_lock)
-    : run_state_(RUNNING),
-      user_lock_(*user_lock),
-      recycling_list_size_(0),
-      allocation_counter_(0) {
-  DCHECK(user_lock);
-}
-
-WinXPCondVar::~WinXPCondVar() {
-  AutoLock auto_lock(internal_lock_);
-  run_state_ = SHUTDOWN;  // Prevent any more waiting.
-
-  DCHECK_EQ(recycling_list_size_, allocation_counter_);
-  if (recycling_list_size_ != allocation_counter_) {  // Rare shutdown problem.
-    // There are threads of execution still in this->TimedWait() and yet the
-    // caller has instigated the destruction of this instance :-/.
-    // A common reason for such "overly hasty" destruction is that the caller
-    // was not willing to wait for all the threads to terminate.  Such hasty
-    // actions are a violation of our usage contract, but we'll give the
-    // waiting thread(s) one last chance to exit gracefully (prior to our
-    // destruction).
-    // Note: waiting_list_ *might* be empty, but recycling is still pending.
-    AutoUnlock auto_unlock(internal_lock_);
-    Broadcast();  // Make sure all waiting threads have been signaled.
-    Sleep(10);  // Give threads a chance to grab internal_lock_.
-    // All contained threads should be blocked on user_lock_ by now :-).
-  }  // Reacquire internal_lock_.
-
-  DCHECK_EQ(recycling_list_size_, allocation_counter_);
-}
-
-void WinXPCondVar::Wait() {
-  // Default to "wait forever" timing, which means have to get a Signal()
-  // or Broadcast() to come out of this wait state.
-  TimedWait(TimeDelta::FromMilliseconds(INFINITE));
-}
-
-void WinXPCondVar::TimedWait(const TimeDelta& max_time) {
-  base::ThreadRestrictions::AssertWaitAllowed();
-  Event* waiting_event;
-  HANDLE handle;
-  {
-    AutoLock auto_lock(internal_lock_);
-    if (RUNNING != run_state_) return;  // Destruction in progress.
-    waiting_event = GetEventForWaiting();
-    handle = waiting_event->handle();
-    DCHECK(handle);
-  }  // Release internal_lock.
-
-  {
-    AutoUnlock unlock(user_lock_);  // Release caller's lock
-    WaitForSingleObject(handle, static_cast<DWORD>(max_time.InMilliseconds()));
-    // Minimize spurious signal creation window by recycling asap.
-    AutoLock auto_lock(internal_lock_);
-    RecycleEvent(waiting_event);
-    // Release internal_lock_
-  }  // Reacquire callers lock to depth at entry.
-}
-
-// Broadcast() is guaranteed to signal all threads that were waiting (i.e., had
-// a cv_event internally allocated for them) before Broadcast() was called.
-void WinXPCondVar::Broadcast() {
-  std::stack<HANDLE> handles;  // See FAQ-question-10.
-  {
-    AutoLock auto_lock(internal_lock_);
-    if (waiting_list_.IsEmpty())
-      return;
-    while (!waiting_list_.IsEmpty())
-      // This is not a leak from waiting_list_.  See FAQ-question 12.
-      handles.push(waiting_list_.PopBack()->handle());
-  }  // Release internal_lock_.
-  while (!handles.empty()) {
-    SetEvent(handles.top());
-    handles.pop();
-  }
-}
-
-// Signal() will select one of the waiting threads, and signal it (signal its
-// cv_event).  For better performance we signal the thread that went to sleep
-// most recently (LIFO).  If we want fairness, then we wake the thread that has
-// been sleeping the longest (FIFO).
-void WinXPCondVar::Signal() {
-  HANDLE handle;
-  {
-    AutoLock auto_lock(internal_lock_);
-    if (waiting_list_.IsEmpty())
-      return;  // No one to signal.
-    // Only performance option should be used.
-    // This is not a leak from waiting_list.  See FAQ-question 12.
-     handle = waiting_list_.PopBack()->handle();  // LIFO.
-  }  // Release internal_lock_.
-  SetEvent(handle);
-}
-
-// GetEventForWaiting() provides a unique cv_event for any caller that needs to
-// wait.  This means that (worst case) we may over time create as many cv_event
-// objects as there are threads simultaneously using this instance's Wait()
-// functionality.
-WinXPCondVar::Event* WinXPCondVar::GetEventForWaiting() {
-  // We hold internal_lock, courtesy of Wait().
-  Event* cv_event;
-  if (0 == recycling_list_size_) {
-    DCHECK(recycling_list_.IsEmpty());
-    cv_event = new Event();
-    cv_event->InitListElement();
-    allocation_counter_++;
-    DCHECK(cv_event->handle());
-  } else {
-    cv_event = recycling_list_.PopFront();
-    recycling_list_size_--;
-  }
-  waiting_list_.PushBack(cv_event);
-  return cv_event;
-}
-
-// RecycleEvent() takes a cv_event that was previously used for Wait()ing, and
-// recycles it for use in future Wait() calls for this or other threads.
-// Note that there is a tiny chance that the cv_event is still signaled when we
-// obtain it, and that can cause spurious signals (if/when we re-use the
-// cv_event), but such is quite rare (see FAQ-question-5).
-void WinXPCondVar::RecycleEvent(Event* used_event) {
-  // We hold internal_lock, courtesy of Wait().
-  // If the cv_event timed out, then it is necessary to remove it from
-  // waiting_list_.  If it was selected by Broadcast() or Signal(), then it is
-  // already gone.
-  used_event->Extract();  // Possibly redundant
-  recycling_list_.PushBack(used_event);
-  recycling_list_size_++;
-}
-//------------------------------------------------------------------------------
-// The next section provides the implementation for the private Event class.
-//------------------------------------------------------------------------------
-
-// Event provides a doubly-linked-list of events for use exclusively by the
-// ConditionVariable class.
-
-// This custom container was crafted because no simple combination of STL
-// classes appeared to support the functionality required.  The specific
-// unusual requirement for a linked-list-class is support for the Extract()
-// method, which can remove an element from a list, potentially for insertion
-// into a second list.  Most critically, the Extract() method is idempotent,
-// turning the indicated element into an extracted singleton whether it was
-// contained in a list or not.  This functionality allows one (or more) of
-// threads to do the extraction.  The iterator that identifies this extractable
-// element (in this case, a pointer to the list element) can be used after
-// arbitrary manipulation of the (possibly) enclosing list container.  In
-// general, STL containers do not provide iterators that can be used across
-// modifications (insertions/extractions) of the enclosing containers, and
-// certainly don't provide iterators that can be used if the identified
-// element is *deleted* (removed) from the container.
-
-// It is possible to use multiple redundant containers, such as an STL list,
-// and an STL map, to achieve similar container semantics.  This container has
-// only O(1) methods, while the corresponding (multiple) STL container approach
-// would have more complex O(log(N)) methods (yeah... N isn't that large).
-// Multiple containers also makes correctness more difficult to assert, as
-// data is redundantly stored and maintained, which is generally evil.
-
-WinXPCondVar::Event::Event() : handle_(0) {
-  next_ = prev_ = this;  // Self referencing circular.
-}
-
-WinXPCondVar::Event::~Event() {
-  if (0 == handle_) {
-    // This is the list holder
-    while (!IsEmpty()) {
-      Event* cv_event = PopFront();
-      DCHECK(cv_event->ValidateAsItem());
-      delete cv_event;
-    }
-  }
-  DCHECK(IsSingleton());
-  if (0 != handle_) {
-    int ret_val = CloseHandle(handle_);
-    DCHECK(ret_val);
-  }
-}
-
-// Change a container instance permanently into an element of a list.
-void WinXPCondVar::Event::InitListElement() {
-  DCHECK(!handle_);
-  handle_ = CreateEvent(NULL, false, false, NULL);
-  DCHECK(handle_);
-}
-
-// Methods for use on lists.
-bool WinXPCondVar::Event::IsEmpty() const {
-  DCHECK(ValidateAsList());
-  return IsSingleton();
-}
-
-void WinXPCondVar::Event::PushBack(Event* other) {
-  DCHECK(ValidateAsList());
-  DCHECK(other->ValidateAsItem());
-  DCHECK(other->IsSingleton());
-  // Prepare other for insertion.
-  other->prev_ = prev_;
-  other->next_ = this;
-  // Cut into list.
-  prev_->next_ = other;
-  prev_ = other;
-  DCHECK(ValidateAsDistinct(other));
-}
-
-WinXPCondVar::Event* WinXPCondVar::Event::PopFront() {
-  DCHECK(ValidateAsList());
-  DCHECK(!IsSingleton());
-  return next_->Extract();
-}
-
-WinXPCondVar::Event* WinXPCondVar::Event::PopBack() {
-  DCHECK(ValidateAsList());
-  DCHECK(!IsSingleton());
-  return prev_->Extract();
-}
-
-// Methods for use on list elements.
-// Accessor method.
-HANDLE WinXPCondVar::Event::handle() const {
-  DCHECK(ValidateAsItem());
-  return handle_;
-}
-
-// Pull an element from a list (if it's in one).
-WinXPCondVar::Event* WinXPCondVar::Event::Extract() {
-  DCHECK(ValidateAsItem());
-  if (!IsSingleton()) {
-    // Stitch neighbors together.
-    next_->prev_ = prev_;
-    prev_->next_ = next_;
-    // Make extractee into a singleton.
-    prev_ = next_ = this;
-  }
-  DCHECK(IsSingleton());
-  return this;
-}
-
-// Method for use on a list element or on a list.
-bool WinXPCondVar::Event::IsSingleton() const {
-  DCHECK(ValidateLinks());
-  return next_ == this;
-}
-
-// Provide pre/post conditions to validate correct manipulations.
-bool WinXPCondVar::Event::ValidateAsDistinct(Event* other) const {
-  return ValidateLinks() && other->ValidateLinks() && (this != other);
-}
-
-bool WinXPCondVar::Event::ValidateAsItem() const {
-  return (0 != handle_) && ValidateLinks();
-}
-
-bool WinXPCondVar::Event::ValidateAsList() const {
-  return (0 == handle_) && ValidateLinks();
-}
-
-bool WinXPCondVar::Event::ValidateLinks() const {
-  // Make sure both of our neighbors have links that point back to us.
-  // We don't do the O(n) check and traverse the whole loop, and instead only
-  // do a local check to (and returning from) our immediate neighbors.
-  return (next_->prev_ == this) && (prev_->next_ == this);
-}
-
-
-/*
-FAQ On WinXPCondVar subtle implementation details:
-
-1) What makes this problem subtle?  Please take a look at "Strategies
-for Implementing POSIX Condition Variables on Win32" by Douglas
-C. Schmidt and Irfan Pyarali.
-http://www.cs.wustl.edu/~schmidt/win32-cv-1.html It includes
-discussions of numerous flawed strategies for implementing this
-functionality.  I'm not convinced that even the final proposed
-implementation has semantics that are as nice as this implementation
-(especially with regard to Broadcast() and the impact on threads that
-try to Wait() after a Broadcast() has been called, but before all the
-original waiting threads have been signaled).
-
-2) Why can't you use a single wait_event for all threads that call
-Wait()?  See FAQ-question-1, or consider the following: If a single
-event were used, then numerous threads calling Wait() could release
-their cs locks, and be preempted just before calling
-WaitForSingleObject().  If a call to Broadcast() was then presented on
-a second thread, it would be impossible to actually signal all
-waiting(?) threads.  Some number of SetEvent() calls *could* be made,
-but there could be no guarantee that those led to to more than one
-signaled thread (SetEvent()'s may be discarded after the first!), and
-there could be no guarantee that the SetEvent() calls didn't just
-awaken "other" threads that hadn't even started waiting yet (oops).
-Without any limit on the number of requisite SetEvent() calls, the
-system would be forced to do many such calls, allowing many new waits
-to receive spurious signals.
-
-3) How does this implementation cause spurious signal events?  The
-cause in this implementation involves a race between a signal via
-time-out and a signal via Signal() or Broadcast().  The series of
-actions leading to this are:
-
-a) Timer fires, and a waiting thread exits the line of code:
-
-    WaitForSingleObject(waiting_event, max_time.InMilliseconds());
-
-b) That thread (in (a)) is randomly pre-empted after the above line,
-leaving the waiting_event reset (unsignaled) and still in the
-waiting_list_.
-
-c) A call to Signal() (or Broadcast()) on a second thread proceeds, and
-selects the waiting cv_event (identified in step (b)) as the event to revive
-via a call to SetEvent().
-
-d) The Signal() method (step c) calls SetEvent() on waiting_event (step b).
-
-e) The waiting cv_event (step b) is now signaled, but no thread is
-waiting on it.
-
-f) When that waiting_event (step b) is reused, it will immediately
-be signaled (spuriously).
-
-
-4) Why do you recycle events, and cause spurious signals?  First off,
-the spurious events are very rare.  They can only (I think) appear
-when the race described in FAQ-question-3 takes place.  This should be
-very rare.  Most(?)  uses will involve only timer expiration, or only
-Signal/Broadcast() actions.  When both are used, it will be rare that
-the race will appear, and it would require MANY Wait() and signaling
-activities.  If this implementation did not recycle events, then it
-would have to create and destroy events for every call to Wait().
-That allocation/deallocation and associated construction/destruction
-would be costly (per wait), and would only be a rare benefit (when the
-race was "lost" and a spurious signal took place). That would be bad
-(IMO) optimization trade-off.  Finally, such spurious events are
-allowed by the specification of condition variables (such as
-implemented in Vista), and hence it is better if any user accommodates
-such spurious events (see usage note in condition_variable.h).
-
-5) Why don't you reset events when you are about to recycle them, or
-about to reuse them, so that the spurious signals don't take place?
-The thread described in FAQ-question-3 step c may be pre-empted for an
-arbitrary length of time before proceeding to step d.  As a result,
-the wait_event may actually be re-used *before* step (e) is reached.
-As a result, calling reset would not help significantly.
-
-6) How is it that the callers lock is released atomically with the
-entry into a wait state?  We commit to the wait activity when we
-allocate the wait_event for use in a given call to Wait().  This
-allocation takes place before the caller's lock is released (and
-actually before our internal_lock_ is released).  That allocation is
-the defining moment when "the wait state has been entered," as that
-thread *can* now be signaled by a call to Broadcast() or Signal().
-Hence we actually "commit to wait" before releasing the lock, making
-the pair effectively atomic.
-
-8) Why do you need to lock your data structures during waiting, as the
-caller is already in possession of a lock?  We need to Acquire() and
-Release() our internal lock during Signal() and Broadcast().  If we tried
-to use a callers lock for this purpose, we might conflict with their
-external use of the lock.  For example, the caller may use to consistently
-hold a lock on one thread while calling Signal() on another, and that would
-block Signal().
-
-9) Couldn't a more efficient implementation be provided if you
-preclude using more than one external lock in conjunction with a
-single ConditionVariable instance?  Yes, at least it could be viewed
-as a simpler API (since you don't have to reiterate the lock argument
-in each Wait() call).  One of the constructors now takes a specific
-lock as an argument, and a there are corresponding Wait() calls that
-don't specify a lock now.  It turns that the resulting implmentation
-can't be made more efficient, as the internal lock needs to be used by
-Signal() and Broadcast(), to access internal data structures.  As a
-result, I was not able to utilize the user supplied lock (which is
-being used by the user elsewhere presumably) to protect the private
-member access.
-
-9) Since you have a second lock, how can be be sure that there is no
-possible deadlock scenario?  Our internal_lock_ is always the last
-lock acquired, and the first one released, and hence a deadlock (due
-to critical section problems) is impossible as a consequence of our
-lock.
-
-10) When doing a Broadcast(), why did you copy all the events into
-an STL queue, rather than making a linked-loop, and iterating over it?
-The iterating during Broadcast() is done so outside the protection
-of the internal lock. As a result, other threads, such as the thread
-wherein a related event is waiting, could asynchronously manipulate
-the links around a cv_event.  As a result, the link structure cannot
-be used outside a lock.  Broadcast() could iterate over waiting
-events by cycling in-and-out of the protection of the internal_lock,
-but that appears more expensive than copying the list into an STL
-stack.
-
-11) Why did the lock.h file need to be modified so much for this
-change?  Central to a Condition Variable is the atomic release of a
-lock during a Wait().  This places Wait() functionality exactly
-mid-way between the two classes, Lock and Condition Variable.  Given
-that there can be nested Acquire()'s of locks, and Wait() had to
-Release() completely a held lock, it was necessary to augment the Lock
-class with a recursion counter. Even more subtle is the fact that the
-recursion counter (in a Lock) must be protected, as many threads can
-access it asynchronously.  As a positive fallout of this, there are
-now some DCHECKS to be sure no one Release()s a Lock more than they
-Acquire()ed it, and there is ifdef'ed functionality that can detect
-nested locks (legal under windows, but not under Posix).
-
-12) Why is it that the cv_events removed from list in Broadcast() and Signal()
-are not leaked?  How are they recovered??  The cv_events that appear to leak are
-taken from the waiting_list_.  For each element in that list, there is currently
-a thread in or around the WaitForSingleObject() call of Wait(), and those
-threads have references to these otherwise leaked events. They are passed as
-arguments to be recycled just aftre returning from WaitForSingleObject().
-
-13) Why did you use a custom container class (the linked list), when STL has
-perfectly good containers, such as an STL list?  The STL list, as with any
-container, does not guarantee the utility of an iterator across manipulation
-(such as insertions and deletions) of the underlying container.  The custom
-double-linked-list container provided that assurance.  I don't believe any
-combination of STL containers provided the services that were needed at the same
-O(1) efficiency as the custom linked list.  The unusual requirement
-for the container class is that a reference to an item within a container (an
-iterator) needed to be maintained across an arbitrary manipulation of the
-container.  This requirement exposes itself in the Wait() method, where a
-waiting_event must be selected prior to the WaitForSingleObject(), and then it
-must be used as part of recycling to remove the related instance from the
-waiting_list.  A hash table (STL map) could be used, but I was embarrased to
-use a complex and relatively low efficiency container when a doubly linked list
-provided O(1) performance in all required operations.  Since other operations
-to provide performance-and/or-fairness required queue (FIFO) and list (LIFO)
-containers, I would also have needed to use an STL list/queue as well as an STL
-map.  In the end I decided it would be "fun" to just do it right, and I
-put so many assertions (DCHECKs) into the container class that it is trivial to
-code review and validate its correctness.
-
-*/
-
-ConditionVariable::ConditionVariable(Lock* user_lock)
-    : impl_(NULL) {
-  static bool use_vista_native_cv = BindVistaCondVarFunctions();
-  if (use_vista_native_cv)
-    impl_= new WinVistaCondVar(user_lock);
-  else
-    impl_ = new WinXPCondVar(user_lock);
-}
-
-ConditionVariable::~ConditionVariable() {
-  delete impl_;
-}
-
-void ConditionVariable::Wait() {
-  impl_->Wait();
-}
-
-void ConditionVariable::TimedWait(const TimeDelta& max_time) {
-  impl_->TimedWait(max_time);
-}
-
 void ConditionVariable::Broadcast() {
-  impl_->Broadcast();
+  WakeAllConditionVariable(&cv_);
 }
 
 void ConditionVariable::Signal() {
-  impl_->Signal();
+  WakeConditionVariable(&cv_);
 }
 
 }  // namespace base