Make ConditionalWeakTable the same as in CoreRT

[mjp41]

There are several concurrency issues in the ConditionalWeakTable, these have been fixed in CoreRT's ConditionWeakTable.

The redesign in CoreRT should be integrated into CoreCLR.

[stephentoub]

It'd be nice to bring over the CoreRT design for other reasons... IIRC, the CoreRT ConditionalWeakTable provides for lock-free reads, whereas the CoreCLR one requires a lock.

[mjp41]

I think it is attempting to do lock-free reads here:

https://github.com/dotnet/coreclr/blob/master/src/mscorlib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs#L238

But I don't think that is correct. The CoreRT version does this correctly.

[stephentoub]

I think it is attempting to do lock-free reads here

No, TryGetValue takes a lock:
https://github.com/dotnet/coreclr/blob/master/src/mscorlib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs#L109

[mjp41]

Sorry, you are correct. I had spent too much time starring at the other. Thank you. I think the version is correct, so I retract my comments on concurrency.

But making them the same would be good. I am planning to address an issue in CoreRT for the ConditionalWeakTable.

[stephentoub]

I'll take a look at porting the corert lock-free reads support back to CoreCLR...