private bool TryAcquireContended(IntPtr currentThreadId, int millisecondsTimeout, bool trackContentions = false)
{
//
// If we already own the lock, just increment the recursion count.
//
if (_owningThreadId == currentThreadId)
{
checked { _recursionCount++; }
return(true);
}
//
// We've already made one lock attempt at this point, so bail early if the timeout is zero.
//
if (millisecondsTimeout == 0)
{
return(false);
}
int spins = 1;
if (s_maxSpinCount == SpinningNotInitialized)
{
// Use RhGetProcessCpuCount directly to avoid Environment.ProcessorCount->ClassConstructorRunner->Lock->Environment.ProcessorCount cycle
s_maxSpinCount = (RuntimeImports.RhGetProcessCpuCount() > 1) ? MaxSpinningValue : SpinningDisabled;
}
while (true)
{
//
// Try to grab the lock. We may take the lock here even if there are existing waiters. This creates the possibility
// of starvation of waiters, but it also prevents lock convoys from destroying perf.
// The starvation issue is largely mitigated by the priority boost the OS gives to a waiter when we set
// the event, after we release the lock. Eventually waiters will be boosted high enough to preempt this thread.
//
int oldState = _state;
if ((oldState & Locked) == 0 && Interlocked.CompareExchange(ref _state, oldState | Locked, oldState) == oldState)
{
goto GotTheLock;
}
//
// Back off by a factor of 2 for each attempt, up to MaxSpinCount
//
if (spins <= s_maxSpinCount)
{
RuntimeImports.RhSpinWait(spins);
spins *= 2;
}
else if (oldState != 0)
{
//
// We reached our spin limit, and need to wait. Increment the waiter count.
// Note that we do not do any overflow checking on this increment. In order to overflow,
// we'd need to have about 1 billion waiting threads, which is inconceivable anytime in the
// forseeable future.
//
int newState = (oldState + WaiterCountIncrement) & ~WaiterWoken;
if (Interlocked.CompareExchange(ref _state, newState, oldState) == oldState)
{
break;
}
}
}
//
// Now we wait.
//
if (trackContentions)
{
Monitor.IncrementLockContentionCount();
}
TimeoutTracker timeoutTracker = TimeoutTracker.Start(millisecondsTimeout);
AutoResetEvent ev = Event;
while (true)
{
Debug.Assert(_state >= WaiterCountIncrement);
bool waitSucceeded = ev.WaitOne(timeoutTracker.Remaining);
while (true)
{
int oldState = _state;
Debug.Assert(oldState >= WaiterCountIncrement);
// Clear the "waiter woken" bit.
int newState = oldState & ~WaiterWoken;
if ((oldState & Locked) == 0)
{
// The lock is available, try to get it.
newState |= Locked;
newState -= WaiterCountIncrement;
if (Interlocked.CompareExchange(ref _state, newState, oldState) == oldState)
{
goto GotTheLock;
}
}
else if (!waitSucceeded)
{
// The lock is not available, and we timed out. We're not going to wait agin.
newState -= WaiterCountIncrement;
if (Interlocked.CompareExchange(ref _state, newState, oldState) == oldState)
{
return(false);
}
}
else
{
// The lock is not available, and we didn't time out. We're going to wait again.
if (Interlocked.CompareExchange(ref _state, newState, oldState) == oldState)
{
break;
}
}
}
}
GotTheLock:
Debug.Assert((_state | Locked) != 0);
Debug.Assert(_owningThreadId == IntPtr.Zero);
Debug.Assert(_recursionCount == 0);
_owningThreadId = currentThreadId;
return(true);
}
