C# ReaderWriterLockSlim 源码
C# ReaderWriterLockSlim 源码
都2020年了，C#读写锁（ReaderWriterLockSlim）的源码都找不到~~~ 只能自己去找了，资源在这，.Net所有代码
.Net Source Code：https://referencesource.microsoft.com/
下面是读写锁 ReaderWriterLockSlim 的源码，之后有时间更新一下源码阅读笔记。
using System.Diagnostics; // for TraceInformation ...
using System.Security.Permissions;
using System.Runtime.CompilerServices;
#if FEATURE_NETCORE
using System.Core; // for strongly typed resources
#endif
namespace System.Threading
{
public enum LockRecursionPolicy
{
NoRecursion = 0,
SupportsRecursion = 1,
}
//
// ReaderWriterCount tracks how many of each kind of lock is held by each thread.
// We keep a linked list for each thread, attached to a ThreadStatic field.
// These are reused wherever possible, so that a given thread will only
// allocate N of these, where N is the maximum number of locks held simultaneously
// by that thread.
//
internal class ReaderWriterCount
{
// Which lock does this object belong to? This is a numeric ID for two reasons:
// 1) We don't want this field to keep the lock object alive, and a WeakReference would
// be too expensive.
// 2) Setting the value of a long is faster than setting the value of a reference.
// The "hot" paths in ReaderWriterLockSlim are short enough that this actually
// matters.
public long lockID;
// How many reader locks does this thread hold on this ReaderWriterLockSlim instance?
public int readercount;
// Ditto for writer/upgrader counts. These are only used if the lock allows recursion.
// But we have to have the fields on every ReaderWriterCount instance, because
// we reuse it for different locks.
public int writercount;
public int upgradecount;
// Next RWC in this thread's list.
public ReaderWriterCount next;
}
/// <summary>
/// A reader-writer lock implementation that is intended to be simple, yet very
/// efficient. In particular only 1 interlocked operation is taken for any lock
/// operation (we use spin locks to achieve this). The spin lock is never held
/// for more than a few instructions (in particular, we never call event APIs
/// or in fact any non-trivial API while holding the spin lock).
/// </summary>
#if !FEATURE_NETCORE
[HostProtection(SecurityAction.LinkDemand, Synchronization=true, ExternalThreading=true)]
#endif
[HostProtection(MayLeakOnAbort = true)]
public class ReaderWriterLockSlim : IDisposable
{
private static readonly int ProcessorCount = Environment.ProcessorCount;
//Specifying if the lock can be reacquired recursively.
private readonly bool _fIsReentrant;
// Lock specification for _spinLock: This lock protects exactly the local fields associated with this
// instance of ReaderWriterLockSlim. It does NOT protect the memory associated with
// the events that hang off this lock (eg writeEvent, readEvent upgradeEvent).
SpinLock _spinLock;
// These variables allow use to avoid Setting events (which is expensive) if we don't have to.
private uint _numWriteWaiters; // maximum number of threads that can be doing a WaitOne on the writeEvent
private uint _numReadWaiters; // maximum number of threads that can be doing a WaitOne on the readEvent
private uint _numWriteUpgradeWaiters; // maximum number of threads that can be doing a WaitOne on the upgradeEvent (at most 1).
private uint _numUpgradeWaiters;
private WaiterStates _waiterStates;
private int _upgradeLockOwnerId;
private int _writeLockOwnerId;
// conditions we wait on.
private EventWaitHandle _writeEvent; // threads waiting to acquire a write lock go here.
private EventWaitHandle _readEvent; // threads waiting to acquire a read lock go here (will be released in bulk)
private EventWaitHandle _upgradeEvent; // thread waiting to acquire the upgrade lock
private EventWaitHandle _waitUpgradeEvent; // thread waiting to upgrade from the upgrade lock to a write lock go here (at most one)
// Every lock instance has a unique ID, which is used by ReaderWriterCount to associate itself with the lock
// without holding a reference to it.
private static long s_nextLockID;
private long _lockID;
// See comments on ReaderWriterCount.
[ThreadStatic]
private static ReaderWriterCount t_rwc;
private bool _fUpgradeThreadHoldingRead;
private const int MaxSpinCount = 20;
//The uint, that contains info like if the writer lock is held, num of
//readers etc.
private uint _owners;
//Various R/W masks
//Note:
//The Uint is divided as follows:
//
//Writer-Owned Waiting-Writers Waiting Upgraders Num-Readers
// 31 30 29 28.......0
//
//Dividing the uint, allows to vastly simplify logic for checking if a
//reader should go in etc. Setting the writer bit will automatically
//make the value of the uint much larger than the max num of readers
//allowed, thus causing the check for max_readers to fail.
private const uint WRITER_HELD = 0x80000000;
private const uint WAITING_WRITERS = 0x40000000;
private const uint WAITING_UPGRADER = 0x20000000;
//The max readers is actually one less then its theoretical max.
//This is done in order to prevent reader count overflows. If the reader
//count reaches max, other readers will wait.
private const uint MAX_READER = 0x10000000 - 2;
private const uint READER_MASK = 0x10000000 - 1;
private bool _fDisposed;
private void InitializeThreadCounts()
{
_upgradeLockOwnerId = -1;
_writeLockOwnerId = -1;
}
public ReaderWriterLockSlim()
: this(LockRecursionPolicy.NoRecursion)
{
}
public ReaderWriterLockSlim(LockRecursionPolicy recursionPolicy)
{
if (recursionPolicy == LockRecursionPolicy.SupportsRecursion)
{
_fIsReentrant = true;
}
InitializeThreadCounts();
_waiterStates = WaiterStates.NoWaiters;
_lockID = Interlocked.Increment(ref s_nextLockID);
}
private bool HasNoWaiters
{
get
{
#if DEBUG
Debug.Assert(_spinLock.IsHeld);
#endif
return (_waiterStates & WaiterStates.NoWaiters) != WaiterStates.None;
}
set
{
#if DEBUG
Debug.Assert(_spinLock.IsHeld);
#endif
if (value)
{
_waiterStates |= WaiterStates.NoWaiters;
}
else
{
_waiterStates &= ~WaiterStates.NoWaiters;
}
}
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static bool IsRWEntryEmpty(ReaderWriterCount rwc)
{
if (rwc.lockID == 0)
return true;
else if (rwc.readercount == 0 && rwc.writercount == 0 && rwc.upgradecount == 0)
return true;
else
return false;
}
private bool IsRwHashEntryChanged(ReaderWriterCount lrwc)
{
return lrwc.lockID != _lockID;
}
/// <summary>
/// This routine retrieves/sets the per-thread counts needed to enforce the
/// various rules related to acquiring the lock.
///
/// DontAllocate is set to true if the caller just wants to get an existing
/// entry for this thread, but doesn't want to add one if an existing one
/// could not be found.
/// </summary>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private ReaderWriterCount GetThreadRWCount(bool dontAllocate)
{
ReaderWriterCount rwc = t_rwc;
ReaderWriterCount empty = null;
while (rwc != null)
{
if (rwc.lockID == _lockID)
return rwc;
if (!dontAllocate && empty == null && IsRWEntryEmpty(rwc))
empty = rwc;
rwc = rwc.next;
}
if (dontAllocate)
return null;
if (empty == null)
{
empty = new ReaderWriterCount();
empty.next = t_rwc;
t_rwc = empty;
}
empty.lockID = _lockID;
return empty;
}
public void EnterReadLock()
{
TryEnterReadLock(-1);
}
//
// Common timeout support
//
private struct TimeoutTracker
{
private int _total;
private int _start;
public TimeoutTracker(TimeSpan timeout)
{
long ltm = (long)timeout.TotalMilliseconds;
if (ltm < -1 || ltm > (long)Int32.MaxValue)
throw new ArgumentOutOfRangeException(nameof(timeout));
_total = (int)ltm;
if (_total != -1 && _total != 0)
_start = Environment.TickCount;
else
_start = 0;
}
public TimeoutTracker(int millisecondsTimeout)
{
if (millisecondsTimeout < -1)
throw new ArgumentOutOfRangeException(nameof(millisecondsTimeout));
_total = millisecondsTimeout;
if (_total != -1 && _total != 0)
_start = Environment.TickCount;
else
_start = 0;
}
public int RemainingMilliseconds
{
get
{
if (_total == -1 || _total == 0)
return _total;
int elapsed = Environment.TickCount - _start;
// elapsed may be negative if TickCount has overflowed by 2^31 milliseconds.
if (elapsed < 0 || elapsed >= _total)
return 0;
return _total - elapsed;
}
}
public bool IsExpired
{
get
{
return RemainingMilliseconds == 0;
}
}
}
public bool TryEnterReadLock(TimeSpan timeout)
{
return TryEnterReadLock(new TimeoutTracker(timeout));
}
public bool TryEnterReadLock(int millisecondsTimeout)
{
return TryEnterReadLock(new TimeoutTracker(millisecondsTimeout));
}
private bool TryEnterReadLock(TimeoutTracker timeout)
{
#if !FEATURE_NETCORE
Thread.BeginCriticalRegion();
#endif // !FEATURE_NETCORE
bool result = false;
try
{
result = TryEnterReadLockCore(timeout);
}
finally
{
#if !FEATURE_NETCORE
if (!result)
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
}
return result;
}
private bool TryEnterReadLockCore(TimeoutTracker timeout)
{
if (_fDisposed)
throw new ObjectDisposedException(null);
ReaderWriterCount lrwc = null;
int id = Environment.CurrentManagedThreadId;
if (!_fIsReentrant)
{
if (id == _writeLockOwnerId)
{
//Check for AW->AR
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_ReadAfterWriteNotAllowed));
}
_spinLock.Enter(EnterSpinLockReason.EnterAnyRead);
lrwc = GetThreadRWCount(false);
//Check if the reader lock is already acquired. Note, we could
//check the presence of a reader by not allocating rwc (But that
//would lead to two lookups in the common case. It's better to keep
//a count in the structure).
if (lrwc.readercount > 0)
{
_spinLock.Exit();
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_RecursiveReadNotAllowed));
}
else if (id == _upgradeLockOwnerId)
{
//The upgrade lock is already held.
//Update the global read counts and exit.
lrwc.readercount++;
_owners++;
_spinLock.Exit();
return true;
}
}
else
{
_spinLock.Enter(EnterSpinLockReason.EnterAnyRead);
lrwc = GetThreadRWCount(false);
if (lrwc.readercount > 0)
{
lrwc.readercount++;
_spinLock.Exit();
return true;
}
else if (id == _upgradeLockOwnerId)
{
//The upgrade lock is already held.
//Update the global read counts and exit.
lrwc.readercount++;
_owners++;
_spinLock.Exit();
_fUpgradeThreadHoldingRead = true;
return true;
}
else if (id == _writeLockOwnerId)
{
//The write lock is already held.
//Update global read counts here,
lrwc.readercount++;
_owners++;
_spinLock.Exit();
return true;
}
}
bool retVal = true;
int spinCount = 0;
for (; ;)
{
// We can enter a read lock if there are only read-locks have been given out
// and a writer is not trying to get in.
if (_owners < MAX_READER)
{
// Good case, there is no contention, we are basically done
_owners++; // Indicate we have another reader
lrwc.readercount++;
break;
}
if (timeout.IsExpired)
{
_spinLock.Exit();
return false;
}
if (spinCount < MaxSpinCount && ShouldSpinForEnterAnyRead())
{
_spinLock.Exit();
spinCount++;
SpinWait(spinCount);
_spinLock.Enter(EnterSpinLockReason.EnterAnyRead);
//The per-thread structure may have been recycled as the lock is acquired (due to message pumping), load again.
if (IsRwHashEntryChanged(lrwc))
lrwc = GetThreadRWCount(false);
continue;
}
// Drat, we need to wait. Mark that we have waiters and wait.
if (_readEvent == null) // Create the needed event
{
LazyCreateEvent(ref _readEvent, EnterLockType.Read);
if (IsRwHashEntryChanged(lrwc))
lrwc = GetThreadRWCount(false);
continue; // since we left the lock, start over.
}
retVal = WaitOnEvent(_readEvent, ref _numReadWaiters, timeout, EnterLockType.Read);
if (!retVal)
{
return false;
}
if (IsRwHashEntryChanged(lrwc))
lrwc = GetThreadRWCount(false);
}
_spinLock.Exit();
return retVal;
}
public void EnterWriteLock()
{
TryEnterWriteLock(-1);
}
public bool TryEnterWriteLock(TimeSpan timeout)
{
return TryEnterWriteLock(new TimeoutTracker(timeout));
}
public bool TryEnterWriteLock(int millisecondsTimeout)
{
return TryEnterWriteLock(new TimeoutTracker(millisecondsTimeout));
}
private bool TryEnterWriteLock(TimeoutTracker timeout)
{
#if !FEATURE_NETCORE
Thread.BeginCriticalRegion();
#endif // !FEATURE_NETCORE
bool result = false;
try
{
result = TryEnterWriteLockCore(timeout);
}
finally
{
#if !FEATURE_NETCORE
if (!result)
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
}
return result;
}
private bool TryEnterWriteLockCore(TimeoutTracker timeout)
{
if (_fDisposed)
throw new ObjectDisposedException(null);
int id = Environment.CurrentManagedThreadId;
ReaderWriterCount lrwc;
bool upgradingToWrite = false;
if (!_fIsReentrant)
{
EnterSpinLockReason enterMyLockReason;
if (id == _writeLockOwnerId)
{
//Check for AW->AW
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_RecursiveWriteNotAllowed));
}
else if (id == _upgradeLockOwnerId)
{
//AU->AW case is allowed once.
upgradingToWrite = true;
enterMyLockReason = EnterSpinLockReason.UpgradeToWrite;
}
else
{
enterMyLockReason = EnterSpinLockReason.EnterWrite;
}
_spinLock.Enter(enterMyLockReason);
lrwc = GetThreadRWCount(true);
//Can't acquire write lock with reader lock held.
if (lrwc != null && lrwc.readercount > 0)
{
_spinLock.Exit();
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_WriteAfterReadNotAllowed));
}
}
else
{
EnterSpinLockReason enterMyLockReason;
if (id == _writeLockOwnerId)
{
enterMyLockReason = EnterSpinLockReason.EnterRecursiveWrite;
}
else if (id == _upgradeLockOwnerId)
{
enterMyLockReason = EnterSpinLockReason.UpgradeToWrite;
}
else
{
enterMyLockReason = EnterSpinLockReason.EnterWrite;
}
_spinLock.Enter(enterMyLockReason);
lrwc = GetThreadRWCount(false);
if (id == _writeLockOwnerId)
{
lrwc.writercount++;
_spinLock.Exit();
return true;
}
else if (id == _upgradeLockOwnerId)
{
upgradingToWrite = true;
}
else if (lrwc.readercount > 0)
{
//Write locks may not be acquired if only read locks have been
//acquired.
_spinLock.Exit();
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_WriteAfterReadNotAllowed));
}
}
bool retVal = true;
int spinCount = 0;
for (; ;)
{
if (IsWriterAcquired())
{
// Good case, there is no contention, we are basically done
SetWriterAcquired();
break;
}
//Check if there is just one upgrader, and no readers.
//Assumption: Only one thread can have the upgrade lock, so the
//following check will fail for all other threads that may sneak in
//when the upgrading thread is waiting.
if (upgradingToWrite)
{
uint readercount = GetNumReaders();
if (readercount == 1)
{
//Good case again, there is just one upgrader, and no readers.
SetWriterAcquired(); // indicate we have a writer.
break;
}
else if (readercount == 2)
{
if (lrwc != null)
{
if (IsRwHashEntryChanged(lrwc))
lrwc = GetThreadRWCount(false);
if (lrwc.readercount > 0)
{
//This check is needed for EU->ER->EW case, as the owner count will be two.
Debug.Assert(_fIsReentrant);
Debug.Assert(_fUpgradeThreadHoldingRead);
//Good case again, there is just one upgrader, and no readers.
SetWriterAcquired(); // indicate we have a writer.
break;
}
}
}
}
if (timeout.IsExpired)
{
_spinLock.Exit();
return false;
}
if (spinCount < MaxSpinCount && ShouldSpinForEnterAnyWrite(upgradingToWrite))
{
_spinLock.Exit();
spinCount++;
SpinWait(spinCount);
_spinLock.Enter(upgradingToWrite ? EnterSpinLockReason.UpgradeToWrite : EnterSpinLockReason.EnterWrite);
continue;
}
if (upgradingToWrite)
{
if (_waitUpgradeEvent == null) // Create the needed event
{
LazyCreateEvent(ref _waitUpgradeEvent, EnterLockType.UpgradeToWrite);
continue; // since we left the lock, start over.
}
Debug.Assert(_numWriteUpgradeWaiters == 0, "There can be at most one thread with the upgrade lock held.");
retVal = WaitOnEvent(_waitUpgradeEvent, ref _numWriteUpgradeWaiters, timeout, EnterLockType.UpgradeToWrite);
//The lock is not held in case of failure.
if (!retVal)
return false;
}
else
{
// Drat, we need to wait. Mark that we have waiters and wait.
if (_writeEvent == null) // create the needed event.
{
LazyCreateEvent(ref _writeEvent, EnterLockType.Write);
continue; // since we left the lock, start over.
}
retVal = WaitOnEvent(_writeEvent, ref _numWriteWaiters, timeout, EnterLockType.Write);
//The lock is not held in case of failure.
if (!retVal)
return false;
}
}
Debug.Assert((_owners & WRITER_HELD) > 0);
if (_fIsReentrant)
{
if (IsRwHashEntryChanged(lrwc))
lrwc = GetThreadRWCount(false);
lrwc.writercount++;
}
_spinLock.Exit();
_writeLockOwnerId = id;
return true;
}
public void EnterUpgradeableReadLock()
{
TryEnterUpgradeableReadLock(-1);
}
public bool TryEnterUpgradeableReadLock(TimeSpan timeout)
{
return TryEnterUpgradeableReadLock(new TimeoutTracker(timeout));
}
public bool TryEnterUpgradeableReadLock(int millisecondsTimeout)
{
return TryEnterUpgradeableReadLock(new TimeoutTracker(millisecondsTimeout));
}
private bool TryEnterUpgradeableReadLock(TimeoutTracker timeout)
{
#if !FEATURE_NETCORE
Thread.BeginCriticalRegion();
#endif // !FEATURE_NETCORE
bool result = false;
try
{
result = TryEnterUpgradeableReadLockCore(timeout);
}
finally
{
#if !FEATURE_NETCORE
if (!result)
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
}
return result;
}
private bool TryEnterUpgradeableReadLockCore(TimeoutTracker timeout)
{
if (_fDisposed)
throw new ObjectDisposedException(null);
int id = Environment.CurrentManagedThreadId;
ReaderWriterCount lrwc;
if (!_fIsReentrant)
{
if (id == _upgradeLockOwnerId)
{
//Check for AU->AU
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_RecursiveUpgradeNotAllowed));
}
else if (id == _writeLockOwnerId)
{
//Check for AU->AW
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_UpgradeAfterWriteNotAllowed));
}
_spinLock.Enter(EnterSpinLockReason.EnterAnyRead);
lrwc = GetThreadRWCount(true);
//Can't acquire upgrade lock with reader lock held.
if (lrwc != null && lrwc.readercount > 0)
{
_spinLock.Exit();
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_UpgradeAfterReadNotAllowed));
}
}
else
{
_spinLock.Enter(EnterSpinLockReason.EnterAnyRead);
lrwc = GetThreadRWCount(false);
if (id == _upgradeLockOwnerId)
{
lrwc.upgradecount++;
_spinLock.Exit();
return true;
}
else if (id == _writeLockOwnerId)
{
//Write lock is already held, Just update the global state
//to show presence of upgrader.
Debug.Assert((_owners & WRITER_HELD) > 0);
_owners++;
_upgradeLockOwnerId = id;
lrwc.upgradecount++;
if (lrwc.readercount > 0)
_fUpgradeThreadHoldingRead = true;
_spinLock.Exit();
return true;
}
else if (lrwc.readercount > 0)
{
//Upgrade locks may not be acquired if only read locks have been
//acquired.
_spinLock.Exit();
throw new LockRecursionException(SR.GetString(SR.LockRecursionException_UpgradeAfterReadNotAllowed));
}
}
bool retVal = true;
int spinCount = 0;
for (; ;)
{
//Once an upgrade lock is taken, it's like having a reader lock held
//until upgrade or downgrade operations are performed.
if ((_upgradeLockOwnerId == -1) && (_owners < MAX_READER))
{
_owners++;
_upgradeLockOwnerId = id;
break;
}
if (timeout.IsExpired)
{
_spinLock.Exit();
return false;
}
if (spinCount < MaxSpinCount && ShouldSpinForEnterAnyRead())
{
_spinLock.Exit();
spinCount++;
SpinWait(spinCount);
_spinLock.Enter(EnterSpinLockReason.EnterAnyRead);
continue;
}
// Drat, we need to wait. Mark that we have waiters and wait.
if (_upgradeEvent == null) // Create the needed event
{
LazyCreateEvent(ref _upgradeEvent, EnterLockType.UpgradeableRead);
continue; // since we left the lock, start over.
}
//Only one thread with the upgrade lock held can proceed.
retVal = WaitOnEvent(_upgradeEvent, ref _numUpgradeWaiters, timeout, EnterLockType.UpgradeableRead);
if (!retVal)
return false;
}
if (_fIsReentrant)
{
//The lock may have been dropped getting here, so make a quick check to see whether some other
//thread did not grab the entry.
if (IsRwHashEntryChanged(lrwc))
lrwc = GetThreadRWCount(false);
lrwc.upgradecount++;
}
_spinLock.Exit();
return true;
}
public void ExitReadLock()
{
ReaderWriterCount lrwc = null;
_spinLock.Enter(EnterSpinLockReason.ExitAnyRead);
lrwc = GetThreadRWCount(true);
if (lrwc == null || lrwc.readercount < 1)
{
//You have to be holding the read lock to make this call.
_spinLock.Exit();
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_MisMatchedRead));
}
if (_fIsReentrant)
{
if (lrwc.readercount > 1)
{
lrwc.readercount--;
_spinLock.Exit();
#if !FEATURE_NETCORE
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
return;
}
if (Environment.CurrentManagedThreadId == _upgradeLockOwnerId)
{
_fUpgradeThreadHoldingRead = false;
}
}
Debug.Assert(_owners > 0, "ReleasingReaderLock: releasing lock and no read lock taken");
--_owners;
Debug.Assert(lrwc.readercount == 1);
lrwc.readercount--;
ExitAndWakeUpAppropriateWaiters();
#if !FEATURE_NETCORE
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
}
public void ExitWriteLock()
{
ReaderWriterCount lrwc;
if (!_fIsReentrant)
{
if (Environment.CurrentManagedThreadId != _writeLockOwnerId)
{
//You have to be holding the write lock to make this call.
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_MisMatchedWrite));
}
_spinLock.Enter(EnterSpinLockReason.ExitAnyWrite);
}
else
{
_spinLock.Enter(EnterSpinLockReason.ExitAnyWrite);
lrwc = GetThreadRWCount(false);
if (lrwc == null)
{
_spinLock.Exit();
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_MisMatchedWrite));
}
if (lrwc.writercount < 1)
{
_spinLock.Exit();
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_MisMatchedWrite));
}
lrwc.writercount--;
if (lrwc.writercount > 0)
{
_spinLock.Exit();
#if !FEATURE_NETCORE
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
return;
}
}
Debug.Assert((_owners & WRITER_HELD) > 0, "Calling ReleaseWriterLock when no write lock is held");
ClearWriterAcquired();
_writeLockOwnerId = -1;
ExitAndWakeUpAppropriateWaiters();
#if !FEATURE_NETCORE
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
}
public void ExitUpgradeableReadLock()
{
ReaderWriterCount lrwc;
if (!_fIsReentrant)
{
if (Environment.CurrentManagedThreadId != _upgradeLockOwnerId)
{
//You have to be holding the upgrade lock to make this call.
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_MisMatchedUpgrade));
}
_spinLock.Enter(EnterSpinLockReason.ExitAnyRead);
}
else
{
_spinLock.Enter(EnterSpinLockReason.ExitAnyRead);
lrwc = GetThreadRWCount(true);
if (lrwc == null)
{
_spinLock.Exit();
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_MisMatchedUpgrade));
}
if (lrwc.upgradecount < 1)
{
_spinLock.Exit();
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_MisMatchedUpgrade));
}
lrwc.upgradecount--;
if (lrwc.upgradecount > 0)
{
_spinLock.Exit();
#if !FEATURE_NETCORE
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
return;
}
_fUpgradeThreadHoldingRead = false;
}
_owners--;
_upgradeLockOwnerId = -1;
ExitAndWakeUpAppropriateWaiters();
#if !FEATURE_NETCORE
Thread.EndCriticalRegion();
#endif // !FEATURE_NETCORE
}
/// <summary>
/// A routine for lazily creating a event outside the lock (so if errors
/// happen they are outside the lock and that we don't do much work
/// while holding a spin lock). If all goes well, reenter the lock and
/// set 'waitEvent'
/// </summary>
private void LazyCreateEvent(ref EventWaitHandle waitEvent, EnterLockType enterLockType)
{
#if DEBUG
Debug.Assert(_spinLock.IsHeld);
Debug.Assert(waitEvent == null);
#endif
_spinLock.Exit();
var newEvent =
new EventWaitHandle(
false,
enterLockType == EnterLockType.Read ? EventResetMode.ManualReset : EventResetMode.AutoReset);
EnterSpinLockReason enterMyLockReason;
switch (enterLockType)
{
case EnterLockType.Read:
case EnterLockType.UpgradeableRead:
enterMyLockReason = EnterSpinLockReason.EnterAnyRead | EnterSpinLockReason.Wait;
break;
case EnterLockType.Write:
enterMyLockReason = EnterSpinLockReason.EnterWrite | EnterSpinLockReason.Wait;
break;
default:
Debug.Assert(enterLockType == EnterLockType.UpgradeToWrite);
enterMyLockReason = EnterSpinLockReason.UpgradeToWrite | EnterSpinLockReason.Wait;
break;
}
_spinLock.Enter(enterMyLockReason);
if (waitEvent == null) // maybe someone snuck in.
waitEvent = newEvent;
else
newEvent.Dispose();
}
/// <summary>
/// Waits on 'waitEvent' with a timeout
/// Before the wait 'numWaiters' is incremented and is restored before leaving this routine.
/// </summary>
private bool WaitOnEvent(
EventWaitHandle waitEvent,
ref uint numWaiters,
TimeoutTracker timeout,
EnterLockType enterLockType)
{
#if DEBUG
Debug.Assert(_spinLock.IsHeld);
#endif
WaiterStates waiterSignaledState = WaiterStates.None;
EnterSpinLockReason enterMyLockReason;
switch (enterLockType)
{
case EnterLockType.UpgradeableRead:
waiterSignaledState = WaiterStates.UpgradeableReadWaiterSignaled;
goto case EnterLockType.Read;
case EnterLockType.Read:
enterMyLockReason = EnterSpinLockReason.EnterAnyRead;
break;
case EnterLockType.Write:
waiterSignaledState = WaiterStates.WriteWaiterSignaled;
enterMyLockReason = EnterSpinLockReason.EnterWrite;
break;
default:
Debug.Assert(enterLockType == EnterLockType.UpgradeToWrite);
enterMyLockReason = EnterSpinLockReason.UpgradeToWrite;
break;
}
// It was not possible to acquire the RW lock because some other thread was holding some type of lock. The other
// thread, when it releases its lock, will wake appropriate waiters. Along with resetting the wait event, clear the
// waiter signaled bit for this type of waiter if applicable, to indicate that a waiter of this type is no longer
// signaled.
//
// If the waiter signaled bit is not updated upon event reset, the following scenario would lead to deadlock:
// - Thread T0 signals the write waiter event or the upgradeable read waiter event to wake a waiter
// - There are no threads waiting on the event, but T1 is in WaitOnEvent() after exiting the spin lock and before
// actually waiting on the event (that is, it's recorded that there is one waiter for the event). It remains in
// this region for a while, in the repro case it typically gets context-switched out.
// - T2 acquires the RW lock in some fashion that blocks T0 or T3 from acquiring the RW lock
// - T0 or T3 fails to acquire the RW lock enough times for it to enter WaitOnEvent for the same event as T1
// - T0 or T3 resets the event
// - T2 releases the RW lock and does not wake a waiter because the reset at the previous step lost a signal but
// _waiterStates was not updated to reflect that
// - T1 and other threads begin waiting on the event, but there's no longer any thread that would wake them
if (waiterSignaledState != WaiterStates.None && (_waiterStates & waiterSignaledState) != WaiterStates.None)
{
_waiterStates &= ~waiterSignaledState;
}
waitEvent.Reset();
numWaiters++;
HasNoWaiters = false;
//Setting these bits will prevent new readers from getting in.
if (_numWriteWaiters == 1)
SetWritersWaiting();
if (_numWriteUpgradeWaiters == 1)
SetUpgraderWaiting();
bool waitSuccessful = false;
_spinLock.Exit(); // Do the wait outside of any lock
try
{
waitSuccessful = waitEvent.WaitOne(timeout.RemainingMilliseconds);
}
finally
{
_spinLock.Enter(enterMyLockReason);
--numWaiters;
if (waitSuccessful &&
waiterSignaledState != WaiterStates.None &&
(_waiterStates & waiterSignaledState) != WaiterStates.None)
{
// Indicate that a signaled waiter of this type has woken. Since non-read waiters are signaled to wake one
// at a time, we avoid waking up more than one waiter of that type upon successive enter/exit loops until
// the signaled thread actually wakes up. For example, if there are multiple write waiters and one thread is
// repeatedly entering and exiting a write lock, every exit would otherwise signal a different write waiter
// to wake up unnecessarily when only one woken waiter may actually succeed in entering the write lock.
_waiterStates &= ~waiterSignaledState;
}
if (_numWriteWaiters == 0 && _numWriteUpgradeWaiters == 0 && _numUpgradeWaiters == 0 && _numReadWaiters == 0)
HasNoWaiters = true;
if (_numWriteWaiters == 0)
ClearWritersWaiting();
if (_numWriteUpgradeWaiters == 0)
ClearUpgraderWaiting();
if (!waitSuccessful) // We may also be about to throw for some reason. Exit myLock.
{
if (enterLockType >= EnterLockType.Write)
{
// Write waiters block read waiters from acquiring the lock. Since this was the last write waiter, try
// to wake up the appropriate read waiters.
ExitAndWakeUpAppropriateReadWaiters();
}
else
{
_spinLock.Exit();
}
}
}
return waitSuccessful;
}
/// <summary>
/// Determines the appropriate events to set, leaves the locks, and sets the events.
/// </summary>
private void ExitAndWakeUpAppropriateWaiters()
{
#if DEBUG
Debug.Assert(_spinLock.IsHeld);
#endif
if (HasNoWaiters)
{
_spinLock.Exit();
return;
}
ExitAndWakeUpAppropriateWaitersPreferringWriters();
}
private void ExitAndWakeUpAppropriateWaitersPreferringWriters()
{
uint readercount = GetNumReaders();
//We need this case for EU->ER->EW case, as the read count will be 2 in
//that scenario.
if (_fIsReentrant)
{
if (_numWriteUpgradeWaiters > 0 && _fUpgradeThreadHoldingRead && readercount == 2)
{
_spinLock.Exit(); // Exit before signaling to improve efficiency (wakee will need the lock)
_waitUpgradeEvent.Set(); // release all upgraders (however there can be at most one).
return;
}
}
if (readercount == 1 && _numWriteUpgradeWaiters > 0)
{
//We have to be careful now, as we are dropping the lock.
//No new writes should be allowed to sneak in if an upgrade
//was pending.
_spinLock.Exit(); // Exit before signaling to improve efficiency (wakee will need the lock)
_waitUpgradeEvent.Set(); // release all upgraders (however there can be at most one).
}
else if (readercount == 0 && _numWriteWaiters > 0)
{
// Check if a waiter of the same type has already been signaled but hasn't woken yet. If so, avoid signaling
// and waking another waiter unnecessarily.
WaiterStates signaled = _waiterStates & WaiterStates.WriteWaiterSignaled;
if (signaled == WaiterStates.None)
{
_waiterStates |= WaiterStates.WriteWaiterSignaled;
}
_spinLock.Exit(); // Exit before signaling to improve efficiency (wakee will need the lock)
if (signaled == WaiterStates.None)
{
_writeEvent.Set(); // release one writer.
}
}
else
{
ExitAndWakeUpAppropriateReadWaiters();
}
}
private void ExitAndWakeUpAppropriateReadWaiters()
{
#if DEBUG
Debug.Assert(_spinLock.IsHeld);
#endif
if (_numWriteWaiters != 0 || _numWriteUpgradeWaiters != 0 || HasNoWaiters)
{
_spinLock.Exit();
return;
}
Debug.Assert(_numReadWaiters != 0 || _numUpgradeWaiters != 0);
bool setReadEvent = _numReadWaiters != 0;
bool setUpgradeEvent = _numUpgradeWaiters != 0 && _upgradeLockOwnerId == -1;
if (setUpgradeEvent)
{
// Check if a waiter of the same type has already been signaled but hasn't woken yet. If so, avoid signaling
// and waking another waiter unnecessarily.
if ((_waiterStates & WaiterStates.UpgradeableReadWaiterSignaled) == WaiterStates.None)
{
_waiterStates |= WaiterStates.UpgradeableReadWaiterSignaled;
}
else
{
setUpgradeEvent = false;
}
}
_spinLock.Exit(); // Exit before signaling to improve efficiency (wakee will need the lock)
if (setReadEvent)
_readEvent.Set(); // release all readers.
if (setUpgradeEvent)
_upgradeEvent.Set(); //release one upgrader.
}
private bool IsWriterAcquired()
{
return (_owners & ~WAITING_WRITERS) == 0;
}
private void SetWriterAcquired()
{
_owners |= WRITER_HELD; // indicate we have a writer.
}
private void ClearWriterAcquired()
{
_owners &= ~WRITER_HELD;
}
private void SetWritersWaiting()
{
_owners |= WAITING_WRITERS;
}
private void ClearWritersWaiting()
{
_owners &= ~WAITING_WRITERS;
}
private void SetUpgraderWaiting()
{
_owners |= WAITING_UPGRADER;
}
private void ClearUpgraderWaiting()
{
_owners &= ~WAITING_UPGRADER;
}
private uint GetNumReaders()
{
return _owners & READER_MASK;
}
private bool ShouldSpinForEnterAnyRead()
{
// If there is a write waiter or write upgrade waiter, the waiter would block a reader from acquiring the RW lock
// because the waiter takes precedence. In that case, the reader is not likely to make progress by spinning.
// Although another thread holding a write lock would prevent this thread from acquiring a read lock, it is by
// itself not a good enough reason to skip spinning.
return HasNoWaiters || (_numWriteWaiters == 0 && _numWriteUpgradeWaiters == 0);
}
private bool ShouldSpinForEnterAnyWrite(bool isUpgradeToWrite)
{
// If there is a write upgrade waiter, the waiter would block a writer from acquiring the RW lock because the waiter
// holds a read lock. In that case, the writer is not likely to make progress by spinning. Regarding upgrading to a
// write lock, there is no type of waiter that would block the upgrade from happening. Although another thread
// holding a read or write lock would prevent this thread from acquiring the write lock, it is by itself not a good
// enough reason to skip spinning.
return isUpgradeToWrite || _numWriteUpgradeWaiters == 0;
}
private static void SpinWait(int spinCount)
{
const int LockSpinCycles = 20;
//Exponential back-off
if ((spinCount < 5) && (ProcessorCount > 1))
{
Thread.SpinWait(LockSpinCycles * spinCount);
}
else
{
Thread.Sleep(0);
}
// Don't want to Sleep(1) in this spin wait:
// - Don't want to spin for that long, since a proper wait will follow when the spin wait fails. The artifical
// delay introduced by Sleep(1) will in some cases be much longer than desired.
// - Sleep(1) would put the thread into a wait state, and a proper wait will follow when the spin wait fails
// anyway, so it's preferable to put the thread into the proper wait state
}
public void Dispose()
{
Dispose(true);
}
private void Dispose(bool disposing)
{
if (disposing && !_fDisposed)
{
if (WaitingReadCount > 0 || WaitingUpgradeCount > 0 || WaitingWriteCount > 0)
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_IncorrectDispose));
if (IsReadLockHeld || IsUpgradeableReadLockHeld || IsWriteLockHeld)
throw new SynchronizationLockException(SR.GetString(SR.SynchronizationLockException_IncorrectDispose));
if (_writeEvent != null)
{
_writeEvent.Dispose();
_writeEvent = null;
}
if (_readEvent != null)
{
_readEvent.Dispose();
_readEvent = null;
}
if (_upgradeEvent != null)
{
_upgradeEvent.Dispose();
_upgradeEvent = null;
}
if (_waitUpgradeEvent != null)
{
_waitUpgradeEvent.Dispose();
_waitUpgradeEvent = null;
}
_fDisposed = true;
}
}
public bool IsReadLockHeld
{
get
{
if (RecursiveReadCount > 0)
return true;
else
return false;
}
}
public bool IsUpgradeableReadLockHeld
{
get
{
if (RecursiveUpgradeCount > 0)
return true;
else
return false;
}
}
public bool IsWriteLockHeld
{
get
{
if (RecursiveWriteCount > 0)
return true;
else
return false;
}
}
public LockRecursionPolicy RecursionPolicy
{
get
{
if (_fIsReentrant)
{
return LockRecursionPolicy.SupportsRecursion;
}
else
{
return LockRecursionPolicy.NoRecursion;
}
}
}
public int CurrentReadCount
{
get
{
int numreaders = (int)GetNumReaders();
if (_upgradeLockOwnerId != -1)
return numreaders - 1;
else
return numreaders;
}
}
public int RecursiveReadCount
{
get
{
int count = 0;
ReaderWriterCount lrwc = GetThreadRWCount(true);
if (lrwc != null)
count = lrwc.readercount;
return count;
}
}
public int RecursiveUpgradeCount
{
get
{
if (_fIsReentrant)
{
int count = 0;
ReaderWriterCount lrwc = GetThreadRWCount(true);
if (lrwc != null)
count = lrwc.upgradecount;
return count;
}
else
{
if (Environment.CurrentManagedThreadId == _upgradeLockOwnerId)
return 1;
else
return 0;
}
}
}
public int RecursiveWriteCount
{
get
{
if (_fIsReentrant)
{
int count = 0;
ReaderWriterCount lrwc = GetThreadRWCount(true);
if (lrwc != null)
count = lrwc.writercount;
return count;
}
else
{
if (Environment.CurrentManagedThreadId == _writeLockOwnerId)
return 1;
else
return 0;
}
}
}
public int WaitingReadCount
{
get
{
return (int)_numReadWaiters;
}
}
public int WaitingUpgradeCount
{
get
{
return (int)_numUpgradeWaiters;
}
}
public int WaitingWriteCount
{
get
{
return (int)_numWriteWaiters;
}
}
private struct SpinLock
{
private int _isLocked;
/// <summary>
/// Used to deprioritize threads attempting to enter the lock when they would not make progress after doing so.
/// <see cref="EnterSpin(EnterSpinLockReason)"/> avoids acquiring the lock as long as the operation for which it
/// was called is deprioritized.
///
/// Layout:
/// - Low 16 bits: Number of threads that have deprioritized an enter-any-write operation
/// - High 16 bits: Number of threads that have deprioritized an enter-any-read operation
/// </summary>
private int _enterDeprioritizationState;
// Layout-specific constants for _enterDeprioritizationState
private const int DeprioritizeEnterAnyReadIncrement = 1 << 16;
private const int DeprioritizeEnterAnyWriteIncrement = 1;
// The variables controlling spinning behavior of this spin lock
private const int LockSpinCycles = 20;
private const int LockSpinCount = 10;
private const int LockSleep0Count = 5;
private const int DeprioritizedLockSleep1Count = 5;
private static int GetEnterDeprioritizationStateChange(EnterSpinLockReason reason)
{
EnterSpinLockReason operation = reason & EnterSpinLockReason.OperationMask;
switch (operation)
{
case EnterSpinLockReason.EnterAnyRead:
return 0;
case EnterSpinLockReason.ExitAnyRead:
// A read lock is held until this thread is able to exit it, so deprioritize enter-write threads as they
// will not be able to make progress
return DeprioritizeEnterAnyWriteIncrement;
case EnterSpinLockReason.EnterWrite:
// Writers are typically much less frequent and much less in number than readers. Waiting writers take
// precedence over new read attempts in order to let current readers release their lock and allow a
// writer to obtain the lock. Before a writer can register as a waiter though, the presence of just
// relatively few enter-read spins can easily starve the enter-write from even entering this lock,
// delaying its spin loop for an unreasonable duration.
//
// Deprioritize enter-read to preference enter-write. This makes it easier for enter-write threads to
// starve enter-read threads. However, writers can already by design starve readers. A waiting writer
// blocks enter-read threads and a new enter-write that needs to wait will be given precedence over
// previously waiting enter-read threads. So this is not a new problem, and the RW lock is designed for
// scenarios where writers are rare compared to readers.
return DeprioritizeEnterAnyReadIncrement;
default:
Debug.Assert(
operation == EnterSpinLockReason.UpgradeToWrite ||
operation == EnterSpinLockReason.EnterRecursiveWrite ||
operation == EnterSpinLockReason.ExitAnyWrite);
// UpgradeToWrite:
// - A read lock is held and an exit-read is not nearby, so deprioritize enter-write threads as they
// will not be able to make progress. This thread also intends to enter a write lock, so deprioritize
// enter -read threads as well, see case EnterSpinLockReason.EnterWrite for the rationale.
// EnterRecursiveWrite, ExitAnyWrite:
// - In both cases, a write lock is held until this thread is able to exit it, so deprioritize
// enter -read and enter-write threads as they will not be able to make progress
return DeprioritizeEnterAnyReadIncrement + DeprioritizeEnterAnyWriteIncrement;
}
}
private ushort EnterForEnterAnyReadDeprioritizedCount
{
get
{
Debug.Assert(DeprioritizeEnterAnyReadIncrement == (1 << 16));
return (ushort)((uint)_enterDeprioritizationState >> 16);
}
}
private ushort EnterForEnterAnyWriteDeprioritizedCount
{
get
{
Debug.Assert(DeprioritizeEnterAnyWriteIncrement == 1);
return (ushort)_enterDeprioritizationState;
}
}
private bool IsEnterDeprioritized(EnterSpinLockReason reason)
{
Debug.Assert((reason & EnterSpinLockReason.Wait) != 0 || reason == (reason & EnterSpinLockReason.OperationMask));
Debug.Assert(
(reason & EnterSpinLockReason.Wait) == 0 ||
(reason & EnterSpinLockReason.OperationMask) == EnterSpinLockReason.EnterAnyRead ||
(reason & EnterSpinLockReason.OperationMask) == EnterSpinLockReason.EnterWrite ||
(reason & EnterSpinLockReason.OperationMask) == EnterSpinLockReason.UpgradeToWrite);
switch (reason)
{
default:
Debug.Assert(
(reason & EnterSpinLockReason.Wait) != 0 ||
reason == EnterSpinLockReason.ExitAnyRead ||
reason == EnterSpinLockReason.EnterRecursiveWrite ||
reason == EnterSpinLockReason.ExitAnyWrite);
return false;
case EnterSpinLockReason.EnterAnyRead:
return EnterForEnterAnyReadDeprioritizedCount != 0;
case EnterSpinLockReason.EnterWrite:
Debug.Assert((GetEnterDeprioritizationStateChange(reason) & DeprioritizeEnterAnyWriteIncrement) == 0);
return EnterForEnterAnyWriteDeprioritizedCount != 0;
case EnterSpinLockReason.UpgradeToWrite:
Debug.Assert((GetEnterDeprioritizationStateChange(reason) & DeprioritizeEnterAnyWriteIncrement) != 0);
return EnterForEnterAnyWriteDeprioritizedCount > 1;
}
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private bool TryEnter()
{
return Interlocked.CompareExchange(ref _isLocked, 1, 0) == 0;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public void Enter(EnterSpinLockReason reason)
{
if (!TryEnter())
{
EnterSpin(reason);
}
}
private void EnterSpin(EnterSpinLockReason reason)
{
int deprioritizationStateChange = GetEnterDeprioritizationStateChange(reason);
if (deprioritizationStateChange != 0)
{
Interlocked.Add(ref _enterDeprioritizationState, deprioritizationStateChange);
}
int processorCount = ProcessorCount;
for (int spinIndex = 0; ; spinIndex++)
{
if (spinIndex < LockSpinCount && processorCount > 1)
{
Thread.SpinWait(LockSpinCycles * (spinIndex + 1)); // Wait a few dozen instructions to let another processor release lock.
}
else if (spinIndex < (LockSpinCount + LockSleep0Count))
{
Thread.Sleep(0); // Give up my quantum.
}
else
{
Thread.Sleep(1); // Give up my quantum.
}
if (!IsEnterDeprioritized(reason))
{
if (_isLocked == 0 && TryEnter())
{
if (deprioritizationStateChange != 0)
{
Interlocked.Add(ref _enterDeprioritizationState, -deprioritizationStateChange);
}
return;
}
continue;
}
// It's possible for an Enter thread to be deprioritized for an extended duration. It's undesirable for a
// deprioritized thread to keep waking up to spin despite a Sleep(1) when a large number of such threads are
// involved. After a threshold of Sleep(1)s, ignore the deprioritization and enter this lock to allow this
// thread to stop spinning and hopefully enter a proper wait state.
Debug.Assert(
reason == EnterSpinLockReason.EnterAnyRead ||
reason == EnterSpinLockReason.EnterWrite ||
reason == EnterSpinLockReason.UpgradeToWrite);
if (spinIndex >= (LockSpinCount + LockSleep0Count + DeprioritizedLockSleep1Count))
{
reason |= EnterSpinLockReason.Wait;
spinIndex = -1;
}
}
}
public void Exit()
{
Debug.Assert(_isLocked != 0, "Exiting spin lock that is not held");
Volatile.Write(ref _isLocked, 0);
}
#if DEBUG
public bool IsHeld => _isLocked != 0;
#endif
}
[Flags]
private enum WaiterStates : byte
{
None = 0x0,
// Used for quick check when there are no waiters
NoWaiters = 0x1,
// Used to avoid signaling more than one waiter to wake up when only one can make progress, see WaitOnEvent
WriteWaiterSignaled = 0x2,
UpgradeableReadWaiterSignaled = 0x4
// Write upgrade waiters are excluded because there can only be one at any given time
}
private enum EnterSpinLockReason
{
EnterAnyRead = 0,
ExitAnyRead = 1,
EnterWrite = 2,
UpgradeToWrite = 3,
EnterRecursiveWrite = 4,
ExitAnyWrite = 5,
OperationMask = 0x7,
Wait = 0x8
}
private enum EnterLockType
{
Read,
UpgradeableRead,
Write,
UpgradeToWrite
}
}
}
