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Optimize locking on AsyncAutoResetEvent #9430

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Merged
michaelstaib merged 1 commit into from
Mar 23, 2026
Merged

Optimize locking on AsyncAutoResetEvent #9430

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119 changes: 66 additions & 53 deletions src/HotChocolate/Core/src/Types/Fetching/AsyncAutoResetEvent.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,69 +5,90 @@ namespace HotChocolate.Fetching;

internal sealed class AsyncAutoResetEvent : INotifyCompletion
{
#if NET9_0_OR_GREATER
private readonly Lock _sync = new();
#else
private readonly object _sync = new();
#endif
private const int Idle = 0;
private const int Signaled = 1;
private const int Waiting = 2;

private int _state;
private Action? _continuation;
private bool _isSignaled;

public bool IsSignaled => Volatile.Read(ref _isSignaled);
public bool IsSignaled => Volatile.Read(ref _state) == Signaled;

public bool IsCompleted => false;
public bool IsCompleted => Volatile.Read(ref _state) == Signaled;

public void GetResult() { }
public void GetResult()
{
// Consume the signal when completing synchronously (via IsCompleted == true).
// CAS failure is benign (e.g. TryResetToIdle cleared it first).
Interlocked.CompareExchange(ref _state, Idle, Signaled);
}

public void OnCompleted(Action continuation)
{
bool wasSignaled;
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;

lock (_sync)
while (true)
{
wasSignaled = _isSignaled;

if (wasSignaled)
{
// consume the signal
_isSignaled = false;
}
else
switch (Volatile.Read(ref _state))
{
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;
}
}
case Idle:
// Register waiter: IDLE -> WAITING
if (Interlocked.CompareExchange(ref _state, Waiting, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

if (wasSignaled)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
case Signaled:
// Consume signal immediately: SIGNALED -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled)
{
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
return;
}
break; // CAS failed, retry

default:
Debug.Fail("OnCompleted called while already waiting.");
return;
}
}
}

public void Set()
{
Action? continuation = null;

lock (_sync)
while (true)
{
if (_continuation is not null)
switch (Volatile.Read(ref _state))
{
// someone is waiting - release them immediately
// we don't set _isSignaled since we're consuming it immediately
continuation = _continuation;
_continuation = null;
}
else
{
// since no one waiting we are storing the signal for the next awaiter
_isSignaled = true;
}
}
case Idle:
// Store signal: IDLE -> SIGNALED
if (Interlocked.CompareExchange(ref _state, Signaled, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

if (continuation is not null)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
case Waiting:
// Wake waiter: WAITING -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Waiting) == Waiting)
{
var c = _continuation!;
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), c, preferLocal: true);
return;
}
break; // CAS failed, retry

case Signaled:
// Already signaled, nothing to do
return;

default:
return;
}
}
}

Expand All @@ -77,15 +98,7 @@ public void Set()
/// </summary>
public bool TryResetToIdle()
{
lock (_sync)
{
if (_continuation is null && _isSignaled)
{
_isSignaled = false;
return true;
}
return false;
}
return Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled;
}

public AsyncAutoResetEvent GetAwaiter() => this;
Expand Down
119 changes: 66 additions & 53 deletions src/HotChocolate/Fusion/src/Fusion.Execution/Execution/AsyncAutoResetEvent.cs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -5,69 +5,90 @@ namespace HotChocolate.Fusion.Execution;

internal sealed class AsyncAutoResetEvent : INotifyCompletion
{
#if NET9_0_OR_GREATER
private readonly Lock _sync = new();
#else
private readonly object _sync = new();
#endif
private const int Idle = 0;
private const int Signaled = 1;
private const int Waiting = 2;

private int _state;
private Action? _continuation;
private bool _isSignaled;

public bool IsSignaled => Volatile.Read(ref _isSignaled);
public bool IsSignaled => Volatile.Read(ref _state) == Signaled;

public bool IsCompleted => false;
public bool IsCompleted => Volatile.Read(ref _state) == Signaled;

public void GetResult() { }
public void GetResult()
{
// Consume the signal when completing synchronously (via IsCompleted == true).
// CAS failure is benign (e.g. TryResetToIdle cleared it first).
Interlocked.CompareExchange(ref _state, Idle, Signaled);
}

public void OnCompleted(Action continuation)
{
bool wasSignaled;
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;

lock (_sync)
while (true)
{
wasSignaled = _isSignaled;

if (wasSignaled)
{
// consume the signal
_isSignaled = false;
}
else
switch (Volatile.Read(ref _state))
{
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;
}
}
case Idle:
// Register waiter: IDLE -> WAITING
if (Interlocked.CompareExchange(ref _state, Waiting, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

if (wasSignaled)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
case Signaled:
// Consume signal immediately: SIGNALED -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled)
{
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
return;
}
break; // CAS failed, retry

default:
Debug.Fail("OnCompleted called while already waiting.");
return;
}
}
}

public void Set()
{
Action? continuation = null;

lock (_sync)
while (true)
{
if (_continuation is not null)
switch (Volatile.Read(ref _state))
{
// someone is waiting - release them immediately
// we don't set _isSignaled since we're consuming it immediately
continuation = _continuation;
_continuation = null;
}
else
{
// since no one waiting we are storing the signal for the next awaiter
_isSignaled = true;
}
}
case Idle:
// Store signal: IDLE -> SIGNALED
if (Interlocked.CompareExchange(ref _state, Signaled, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

if (continuation is not null)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
case Waiting:
// Wake waiter: WAITING -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Waiting) == Waiting)
{
var c = _continuation!;
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), c, preferLocal: true);
return;
}
break; // CAS failed, retry

case Signaled:
// Already signaled, nothing to do
return;

default:
return;
}
}
}

Expand All @@ -77,15 +98,7 @@ public void Set()
/// </summary>
public bool TryResetToIdle()
{
lock (_sync)
{
if (_continuation is null && _isSignaled)
{
_isSignaled = false;
return true;
}
return false;
}
return Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled;
}

public AsyncAutoResetEvent GetAwaiter() => this;
Expand Down
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