/// <summary>
/// This method lazily initializes the event object. It uses CAS to guarantee that
/// many threads racing to call this at once don't result in more than one event
/// being stored and used. The event will be signaled or unsignaled depending on
/// the state of the thin-event itself, with synchronization taken into account.
/// </summary>
/// <returns>True if a new event was created and stored, false otherwise.</returns>
private bool LazyInitializeEvent()
{
int preInitializeState = m_state;
ManualResetEvent newEventObj = new ManualResetEvent(preInitializeState == EVENT_SIGNALED);
// We have to CAS this in case we are racing with another thread. We must
// guarantee only one event is actually stored in this field.
if (Interlocked.CompareExchange(ref m_eventObj, newEventObj, null) != null)
{
// We raced with someone else and lost. Destroy the garbage event.
newEventObj.Close();
return(false);
}
else
{
#if DEBUG
TraceHelpers.TraceInfo("{0} - tid {1}: ManualResetEventSlim::LazyInitializeEvent - created a real event", m_id, Thread.CurrentThread.ManagedThreadId);
#endif
// Now that the event is published, verify that the state hasn't changed since
// we snapped the preInitializeState. Another thread could have done that
// between our initial observation above and here. The barrier incurred from
// the CAS above (in addition to m_state being volatile) prevents this read
// from moving earlier and being collapsed with our original one.
if (m_state != preInitializeState)
{
TraceHelpers.Assert(IsSet,
"the only safe concurrent transition is from unset->set: detected set->unset");
// We saw it as unsignaled, but it has since become set.
m_eventObj.Set();
}
return(true);
}
}
/// <summary>
/// WaitAny simulates a Win32-style WaitAny on the set of thin-events.
/// </summary>
/// <param name="events">An array of thin-events (null elements permitted)</param>
/// <returns>The index of the specific event in events that caused us to wake up.</returns>
internal static int WaitAny(ManualResetEventSlim[] events)
{
TraceHelpers.Assert(events != null);
#if DEBUG
string eventList = "";
foreach (ManualResetEventSlim e in events)
{
if (e != null)
{
eventList = eventList + e.m_id + ", ";
}
}
TraceHelpers.TraceInfo("{0}: ManualResetEventSlim::WaitAny - waiting on {1} events ({2})",
Thread.CurrentThread.ManagedThreadId, events.Length, eventList);
#endif
// First, ensure all the events have been set up.
int nullEvents = 0;
for (int i = 0; i < events.Length; i++)
{
// We permit nulls in the array -- we just skip them.
if (events[i] == null)
{
nullEvents++;
continue;
}
// Throw an exception if the event has been disposed.
events[i].ThrowIfDisposed();
// If this event has been set, avoid waiting altogether.
if (events[i].IsSet)
{
return(i);
}
// Otherwise, lazily initialize the event in preparation for waiting.
if (events[i].m_eventObj == null)
{
events[i].LazyInitializeEvent();
}
}
// Lastly, accumulate the events in preparation for a true wait.
WaitHandle[] waitHandles = new WaitHandle[events.Length - nullEvents];
TraceHelpers.Assert(waitHandles.Length > 0);
for (int i = 0, j = 0; i < events.Length; i++)
{
if (events[i] == null)
{
continue;
}
waitHandles[j] = events[i].WaitHandle;
j++;
}
// And finally, issue the real wait.
int index = WaitHandle.WaitAny(waitHandles);
// Translate this back into the events array index. The 'waitHandles' array
// will effectively have the non-null elements "slid down" into the positions
// from 'events' that contain nulls. We count the number of null handles before
// the index and add that to get our real position.
for (int i = 0, j = -1; i < events.Length; i++)
{
// If the current event is non-null, increment our translation index.
if (events[i] != null)
{
j++;
// If we found the element, adjust our index and break.
if (j == index)
{
index = i;
break;
}
}
TraceHelpers.Assert(i != events.Length - 1, "didn't find a non-null event");
}
TraceHelpers.Assert(events[index] != null, "expected non-null event");
return(index);
}
/// <summary>
/// Waits for the event to become set. We will spin briefly if the event isn't set
/// (assuming a non-0 timeout), before falling back to a true wait.
/// </summary>
/// <param name="millisecondsTimeout">The maximum amount of time to wait.</param>
/// <returns>True if the wait succeeded, false if the timeout expired.</returns>
/// <exception cref="System.ArgumentOutOfRangeException">If the timeout is not within range.</exception>
public bool Wait(int millisecondsTimeout)
{
ThrowIfDisposed();
if (millisecondsTimeout < -1)
{
throw Error.ArgumentOutOfRange("millisecondsTimeout");
}
if (!IsSet)
{
if (millisecondsTimeout == 0)
{
// For 0-timeouts, we just return immediately.
return(false);
}
else
{
#if DEBUG
TraceHelpers.TraceInfo("{0} - tid {1}: ManualResetEventSlim::Wait - doing a spin wait for {2} spins",
m_id, Thread.CurrentThread.ManagedThreadId, SpinCount);
#endif
// We spin briefly before falling back to allocating and/or waiting on a true event.
SpinWait s = new SpinWait();
Stopwatch sw = null;
if (SpinCount > 0)
{
if (millisecondsTimeout != Timeout.Infinite)
{
// We will account for time spent spinning, so that we can decrement it from our
// timeout. In most cases the time spent in this section will be negligible. But
// we can't discount the possibility of our thread being switched out for a lengthy
// period of time. The timeout adjustments only take effect when and if we actually
// decide to block in the kernel below.
sw = Stopwatch.StartNew();
}
for (int i = 0; i < SpinCount; i++)
{
s.SpinOnce();
// Keep rechecking the state. If we've become signaled while we spun above,
// return. This is done with a volatile read to prevent reordering and hoisting
// out of the loop.
if (IsSet)
{
return(true);
}
}
}
if (m_eventObj == null)
{
// Lazily allocate the event. This method internally handles races w/ other threads.
LazyInitializeEvent();
if (IsSet)
{
// If it has since become signaled, there's no need to wait. This is strictly
// unnecessary, but does avoid a kernel transition. Since the allocation could
// have taken some time, this might be worth the extra check in some cases.
return(true);
}
}
#if DEBUG
TraceHelpers.TraceInfo("{0} - tid {1}: ManualResetEventSlim::Wait - entering a real wait",
m_id, Thread.CurrentThread.ManagedThreadId);
#endif
// Do our dynamic spin count accounting. We don't worry about thread safety here.
// It's OK for reads and writes to be carried out non-atomically -- we might miss one
// here or there, but because it's a 4-byte aligned value we are guaranteed no tears.
if (UseDynamicSpinAdjustment)
{
int spinCount = SpinCount;
if (spinCount != 0 && spinCount < MAX_DYNAMIC_SPIN)
{
// We use an algorithm similar to the OS critical section. If our
// spinning didn't work out, we increment the counter so that we increase
// the chances of spinning working the next time.
m_spinCount = ((spinCount + 1) % int.MaxValue) | USE_DYNAMIC_SPIN_MASK;
}
else
{
// If we reached the dynamic spin maximum, and it's still not working, bail.
// We don't want to spin any longer since it's not buying us anything (in fact,
// it's just wasting time). This forces us to go straight to the wait next time.
m_spinCount = 0;
}
}
// If we spun at all, we'll take into account the time elapsed by adjusting
// the timeout value before blocking in the kernel.
int realMillisecondsTimeout = millisecondsTimeout;
if (sw != null)
{
long elapsedMilliseconds = sw.ElapsedMilliseconds;
if (elapsedMilliseconds > int.MaxValue)
{
return(false);
}
TraceHelpers.Assert(elapsedMilliseconds >= 0);
realMillisecondsTimeout -= (int)elapsedMilliseconds;
if (realMillisecondsTimeout <= 0)
{
return(false);
}
}
return(m_eventObj.WaitOne(realMillisecondsTimeout, false));
}
}
return(true);
}
