EM_Task/CoreUObject/Private/UObject/GCScopeLock.h

// Copyright Epic Games, Inc. All Rights Reserved.

/*=============================================================================
        Garbage Collection scope lock.
=============================================================================*/

#pragma once

#include "CoreMinimal.h"
#include "HAL/ThreadSafeCounter.h"
#include "HAL/PlatformProcess.h"
#include "Misc/ScopeLock.h"
#include "HAL/ThreadSafeBool.h"
#include "HAL/Event.h"

/**
 * Garbage Collection synchronization objects
 * Will not lock other threads if GC is not running.
 * Has the ability to only lock for GC if no other locks are present.
 */
class FGCCSyncObject
{
    /** Non zero if any of the non-game threads is blocking GC */
    FThreadSafeCounter AsyncCounter;
    /** Non zero if GC is running */
    FThreadSafeCounter GCCounter;
    /** Non zero if GC wants to run but is blocked by some other thread \
        This flag is not automatically enforced on the async threads, instead
                    threads have to manually implement support for it. */
    TAtomic<int32> GCWantsToRunCounter{0};
    /** Critical section for thread safe operations */
    FCriticalSection Critical;
    /** Event used to block non-game threads when GC is running */
    FEvent* GCUnlockedEvent;

public:
    FGCCSyncObject();
    ~FGCCSyncObject();

    /** Creates the singleton object */
    static void Create();

    /** Gets the singleton object */
    static FGCCSyncObject& Get();

    /** Lock on non-game thread. Will block if GC is running. */
    void LockAsync()
    {
        if (!IsInGameThread())
        {
            // Wait until GC is done if it was running when entering this function
            bool bLocked = false;
            do
            {
                if (GCCounter.GetValue() > 0)
                {
                    GCUnlockedEvent->Wait();
                }
                {
                    FScopeLock CriticalLock(&Critical);
                    if (GCCounter.GetValue() == 0)
                    {
                        AsyncCounter.Increment();
                        bLocked = true;
                    }
                }
            } while (!bLocked);
        }
    }
    /** Release lock from non-game thread */
    void UnlockAsync()
    {
        if (!IsInGameThread())
        {
            AsyncCounter.Decrement();
        }
    }
    /** Lock for GC. Will block if any other thread has locked. */
    void GCLock()
    {
        // Signal other threads that GC wants to run
        SetGCIsWaiting();

        // Wait until all other threads are done if they're currently holding the lock
        bool bLocked = false;
        do
        {
            FPlatformProcess::ConditionalSleep([&]()
                                               {
                                                   return AsyncCounter.GetValue() == 0;
                                               });
            {
                FScopeLock CriticalLock(&Critical);
                if (AsyncCounter.GetValue() == 0)
                {
                    GCUnlockedEvent->Reset();
                    int32 GCCounterValue = GCCounter.Increment();
                    check(GCCounterValue == 1);  // GCLock doesn't support recursive locks
                    // At this point GC can run so remove the signal that it's waiting
                    FPlatformMisc::MemoryBarrier();
                    ResetGCIsWaiting();
                    bLocked = true;
                }
            }
        } while (!bLocked);
    }
    /** Checks if any async thread has a lock */
    bool IsAsyncLocked() const
    {
        return AsyncCounter.GetValue() != 0;
    }
    /** Checks if GC has a lock */
    bool IsGCLocked() const
    {
        return GCCounter.GetValue() != 0;
    }
    /** Lock for GC. Will not block and return false if any other thread has already locked. */
    bool TryGCLock()
    {
        bool bSuccess = false;
        FScopeLock CriticalLock(&Critical);
        // If any other thread is currently locking we just exit
        if (AsyncCounter.GetValue() == 0)
        {
            GCUnlockedEvent->Reset();
            int32 GCCounterValue = GCCounter.Increment();
            check(GCCounterValue == 1);  // GCLock doesn't support recursive locks
            bSuccess = true;
        }
        return bSuccess;
    }
    /** Unlock GC */
    void GCUnlock()
    {
        GCUnlockedEvent->Trigger();
        GCCounter.Decrement();
    }

    /** Manually mark GC state as 'waiting to run' */
    void SetGCIsWaiting()
    {
        GCWantsToRunCounter++;
    }

    /** Manually reset GC 'waiting to run' state*/
    void ResetGCIsWaiting()
    {
        GCWantsToRunCounter.Store(0);
    }

    /** True if GC wants to run on the game thread but is maybe blocked by some other thread */
    FORCEINLINE bool IsGCWaiting() const
    {
        return !!GCWantsToRunCounter.Load(EMemoryOrder::Relaxed);
    }
};