public bool TryEnterWriteLock(int millisecondsTimeout)
{
var currentThreadState = CurrentThreadState;
if (CheckState(currentThreadState, millisecondsTimeout, LockState.Write))
{
++currentThreadState.WriterRecursiveCount;
return(true);
}
++WaitingWriteCount;
var isUpgradable = (currentThreadState.LockState & LockState.Upgradable) > 0;
var registered = false;
var success = false;
RuntimeHelpers.PrepareConstrainedRegions();
try
{
/* If the code goes there that means we had a read lock beforehand
* that need to be suppressed, we also take the opportunity to register
* our interest in the write lock to avoid other write wannabe process
* coming in the middle
*/
if (isUpgradable && _rwLock >= _rwRead)
{
try
{
// Empty
}
finally
{
if (Interlocked.Add(ref _rwLock, _rwWaitUpgrade - _rwRead) >> _rwReadBit == 0)
{
_readerDoneEvent.Set();
}
registered = true;
}
}
var stateCheck = isUpgradable ? _rwWaitUpgrade + _rwWait : _rwWait;
var start = millisecondsTimeout == -1 ? 0 : _stopwatch.ElapsedMilliseconds;
var registration = isUpgradable ? _rwWaitUpgrade : _rwWait;
do
{
var state = _rwLock;
if (state <= stateCheck)
{
try
{
// Empty
}
finally
{
var toWrite = state + _rwWrite - (registered ? registration : 0);
if (Interlocked.CompareExchange(ref _rwLock, toWrite, state) == state)
{
_writerDoneEvent.Reset();
currentThreadState.LockState ^= LockState.Write;
++currentThreadState.WriterRecursiveCount;
--WaitingWriteCount;
registered = false;
success = true;
}
}
if (success)
{
return(true);
}
}
state = _rwLock;
// We register our interest in taking the Write lock (if upgradeable it's already done)
if (!isUpgradable)
{
while ((state & _rwWait) == 0)
{
try
{
// Empty
}
finally
{
registered |= Interlocked.CompareExchange(ref _rwLock, state | _rwWait, state) == state;
}
if (registered)
{
break;
}
state = _rwLock;
}
}
// Before falling to sleep
do
{
if (_rwLock <= stateCheck)
{
break;
}
if ((_rwLock & _rwWrite) != 0)
{
_writerDoneEvent.Wait(ComputeTimeout(millisecondsTimeout, start));
}
else if (_rwLock >> _rwReadBit > 0)
{
_readerDoneEvent.Wait(ComputeTimeout(millisecondsTimeout, start));
}
} while (millisecondsTimeout < 0 || _stopwatch.ElapsedMilliseconds - start < millisecondsTimeout);
} while (millisecondsTimeout < 0 || _stopwatch.ElapsedMilliseconds - start < millisecondsTimeout);
--WaitingWriteCount;
}
finally
{
if (registered)
{
Interlocked.Add(ref _rwLock, isUpgradable ? -_rwWaitUpgrade : -_rwWait);
}
}
return(false);
}
private bool TryEnterReadLock(int millisecondsTimeout, ref bool success)
{
var currentThreadState = CurrentThreadState;
if (CheckState(currentThreadState, millisecondsTimeout, LockState.Read))
{
++currentThreadState.ReaderRecursiveCount;
return(true);
}
// This is downgrading from upgradable, no need for check since
// we already have a sort-of read lock that's going to disappear
// after user calls ExitUpgradeableReadLock.
// Same idea when recursion is allowed and a write thread wants to
// go for a Read too.
if ((currentThreadState.LockState & LockState.Upgradable) > 0 || (!_noRecursion && (currentThreadState.LockState & LockState.Write) > 0))
{
RuntimeHelpers.PrepareConstrainedRegions();
try
{
// Empty
}
finally
{
Interlocked.Add(ref _rwLock, _rwRead);
currentThreadState.LockState |= LockState.Read;
++currentThreadState.ReaderRecursiveCount;
success = true;
}
return(true);
}
WaitingReadCount++;
var start = millisecondsTimeout == -1 ? 0 : _stopwatch.ElapsedMilliseconds;
do
{
/* Check if a writer is present (RwWrite) or if there is someone waiting to
* acquire a writer lock in the queue (RwWait | RwWaitUpgrade).
*/
if ((_rwLock & (_rwWrite | _rwWait | _rwWaitUpgrade)) > 0)
{
_writerDoneEvent.Wait(ComputeTimeout(millisecondsTimeout, start));
continue;
}
/* Optimistically try to add ourselves to the reader value
* if the adding was too late and another writer came in between
* we revert the operation.
*/
RuntimeHelpers.PrepareConstrainedRegions();
try
{
// Empty
}
finally
{
int add;
if (((add = Interlocked.Add(ref _rwLock, _rwRead)) & (_rwWrite | _rwWait | _rwWaitUpgrade)) == 0)
{
/* If we are the first reader, reset the event to let other threads
* sleep correctly if they try to acquire write lock
*/
if (add >> _rwReadBit == 1)
{
_readerDoneEvent.Reset();
}
currentThreadState.LockState ^= LockState.Read;
++currentThreadState.ReaderRecursiveCount;
--WaitingReadCount;
success = true;
}
else
{
Interlocked.Add(ref _rwLock, -_rwRead);
}
}
if (success)
{
return(true);
}
_writerDoneEvent.Wait(ComputeTimeout(millisecondsTimeout, start));
} while (millisecondsTimeout == -1 || _stopwatch.ElapsedMilliseconds - start < millisecondsTimeout);
--WaitingReadCount;
return(false);
}
/// <summary>
/// Try acquire the lock with long path, this is usually called after the first path in Enter and
/// TryEnter failed The reason for short path is to make it inline in the run time which improves the
/// performance. This method assumed that the parameter are validated in Enter or TryEnter method.
/// </summary>
/// <param name="millisecondsTimeout">The timeout milliseconds</param>
/// <param name="lockTaken">The lockTaken param</param>
private void ContinueTryEnter(int millisecondsTimeout, ref bool lockTaken)
{
// The fast path doesn't throw any exception, so we have to validate the parameters here
if (lockTaken)
{
lockTaken = false;
throw new ArgumentException(SR.SpinLock_TryReliableEnter_ArgumentException);
}
if (millisecondsTimeout < -1)
{
throw new ArgumentOutOfRangeException(
nameof(millisecondsTimeout), millisecondsTimeout, SR.SpinLock_TryEnter_ArgumentOutOfRange);
}
uint startTime = 0;
if (millisecondsTimeout != Timeout.Infinite && millisecondsTimeout != 0)
{
startTime = TimeoutHelper.GetTime();
}
if (IsThreadOwnerTrackingEnabled)
{
// Slow path for enabled thread tracking mode
ContinueTryEnterWithThreadTracking(millisecondsTimeout, startTime, ref lockTaken);
return;
}
// then thread tracking is disabled
// In this case there are three ways to acquire the lock
// 1- the first way the thread either tries to get the lock if it's free or updates the waiters, if the turn >= the processors count then go to 3 else go to 2
// 2- In this step the waiter threads spins and tries to acquire the lock, the number of spin iterations and spin count is dependent on the thread turn
// the late the thread arrives the more it spins and less frequent it check the lock availability
// Also the spins count is increases each iteration
// If the spins iterations finished and failed to acquire the lock, go to step 3
// 3- This is the yielding step, there are two ways of yielding Thread.Yield and Sleep(1)
// If the timeout is expired in after step 1, we need to decrement the waiters count before returning
int observedOwner;
int turn = int.MaxValue;
// ***Step 1, take the lock or update the waiters
// try to acquire the lock directly if possible or update the waiters count
observedOwner = _owner;
if ((observedOwner & LOCK_ANONYMOUS_OWNED) == LOCK_UNOWNED)
{
if (CompareExchange(ref _owner, observedOwner | 1, observedOwner, ref lockTaken) == observedOwner)
{
// Acquired lock
return;
}
if (millisecondsTimeout == 0)
{
// Did not acquire lock in CompareExchange and timeout is 0 so fail fast
return;
}
}
else if (millisecondsTimeout == 0)
{
// Did not acquire lock as owned and timeout is 0 so fail fast
return;
}
else // failed to acquire the lock, then try to update the waiters. If the waiters count reached the maximum, just break the loop to avoid overflow
{
if ((observedOwner & WAITERS_MASK) != MAXIMUM_WAITERS)
{
// This can still overflow, but maybe there will never be that many waiters
turn = (Interlocked.Add(ref _owner, 2) & WAITERS_MASK) >> 1;
}
}
// lock acquired failed and waiters updated
// *** Step 2, Spinning and Yielding
SpinWait spinner = default;
if (turn > Environment.ProcessorCount)
{
spinner.Count = SpinWait.YieldThreshold;
}
while (true)
{
spinner.SpinOnce(SLEEP_ONE_FREQUENCY);
observedOwner = _owner;
if ((observedOwner & LOCK_ANONYMOUS_OWNED) == LOCK_UNOWNED)
{
int newOwner = (observedOwner & WAITERS_MASK) == 0 ? // Gets the number of waiters, if zero
observedOwner | 1 // don't decrement it. just set the lock bit, it is zero because a previous call of Exit(false) which corrupted the waiters
: (observedOwner - 2) | 1; // otherwise decrement the waiters and set the lock bit
Debug.Assert((newOwner & WAITERS_MASK) >= 0);
if (CompareExchange(ref _owner, newOwner, observedOwner, ref lockTaken) == observedOwner)
{
return;
}
}
if (spinner.Count % TIMEOUT_CHECK_FREQUENCY == 0)
{
// Check the timeout.
if (millisecondsTimeout != Timeout.Infinite && TimeoutHelper.UpdateTimeOut(startTime, millisecondsTimeout) <= 0)
{
DecrementWaiters();
return;
}
}
}
}
