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Revert "Use a ReaderWriterLockSlim in RcwCache" #120370

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Merged
merged 1 commit into from
Oct 3, 2025
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Revert "Use a ReaderWriterLockSlim in RcwCache" #120370

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23 changes: 4 additions & 19 deletions src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/ComWrappers.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1289,7 +1289,7 @@ private static void AddWrapperToReferenceTrackerHandleCache(NativeObjectWrapper

private sealed class RcwCache
{
private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();
private readonly Lock _lock = new Lock(useTrivialWaits: true);
private readonly Dictionary<IntPtr, GCHandle> _cache = [];

/// <summary>
Expand All @@ -1301,8 +1301,7 @@ private sealed class RcwCache
/// <returns>The proxy object currently in the cache for <paramref name="comPointer"/> or the proxy object owned by <paramref name="wrapper"/> if no entry exists and the corresponding native wrapper.</returns>
public (NativeObjectWrapper actualWrapper, object actualProxy) GetOrAddProxyForComInstance(IntPtr comPointer, NativeObjectWrapper wrapper, object comProxy)
{
_lock.EnterWriteLock();
try
lock (_lock)
{
Debug.Assert(wrapper.ProxyHandle.Target == comProxy);
ref GCHandle rcwEntry = ref CollectionsMarshal.GetValueRefOrAddDefault(_cache, comPointer, out bool exists);
Expand Down Expand Up @@ -1337,16 +1336,11 @@ private sealed class RcwCache
// Return our target object.
return (wrapper, comProxy);
}
finally
{
_lock.ExitWriteLock();
}
}

public object? FindProxyForComInstance(IntPtr comPointer)
{
_lock.EnterReadLock();
try
lock (_lock)
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@rhuijben rhuijben Oct 3, 2025 •
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Here a readlock is not enough. It updates the hashtable under some circumstances, which may go concurrent under a read lock.

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@rhuijben rhuijben Oct 3, 2025 •
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Upgrading to a write lock before the update (_cache.Remove()) may help too... But the single lock is probably just as easy.

{
if (_cache.TryGetValue(comPointer, out GCHandle existingHandle))
{
Expand All @@ -1363,16 +1357,11 @@ private sealed class RcwCache

return null;
}
finally
{
_lock.ExitReadLock();
}
}

public void Remove(IntPtr comPointer, NativeObjectWrapper wrapper)
{
_lock.EnterWriteLock();
try
lock (_lock)
{
// TryGetOrCreateObjectForComInstanceInternal may have put a new entry into the cache
// in the time between the GC cleared the contents of the GC handle but before the
Expand All @@ -1387,10 +1376,6 @@ public void Remove(IntPtr comPointer, NativeObjectWrapper wrapper)
cachedRef.Free();
}
}
finally
{
_lock.ExitWriteLock();
}
}
}

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