RFR: 8288140: Avoid redundant Hashtable.get call in Signal.handle [v2]

[Peter Levart]

On Sat, 11 Jun 2022 07:55:52 GMT, Peter Levart <plevart at openjdk.org> wrote:

>> Oops, yes you are correct!
>
> Hi,
> I think the synchronized block was redundant already in original code. Since the entire handle method is `static synchronized` and it is the only method that modifies the `handlers` and `signals` maps.
> But even with so much redundant synchronization, the Signal class is not without races. There are multiple possible races in `dispatch(int number)` method which is called from native code to dispatch a signal:
> - race no. 1: dispatch method (not synchronized) performs 2 independent lookups into `signals` and `handlers` maps respectively, assuming their inter-referenced state is consistent. But `handle` method may be concurrently modifying them, so `dispatch` may see updated state of `signals` map while seeing old state of `handlers` map or vice versa.
> - race  no. 2: besides `signals` and `handlers` there is a 3rd map in native code, updated with `handle0` native method. Native code dispatches signals according to that native map, forwarding them to either native handlers or to `dispatch` Java method. But `handle` method may be modifying that native map concurrently, so dispatch may be called as a consequence of updated native map while seeing old states of `signals` and `handlers` maps.
> 
> I'm sure I might have missed some additional races.
> 
> How to fix this? Is it even necessary? I think that this internal API is not used frequently so this was hardly an issue. But anyway, it would be a challenge. While the two java maps: `handlers` and `signals` could be replaced with a single map, there is the 3rd map in native code. We would not want to make `dispatch` method synchronized, so with careful ordering of modifications it is perhaps possible to account for races and make them harmless...
> WDYT?

Well, studying this further, I think some races are already accounted for and are harmless except one. The `handle` method 1st attempts to register handler in native code via call to `handle0` and then updates the `signals` map.. As soon as native code registers the handler, `dispatch` may be called and the lookup into `signals` map may return null which would cause NPE in the following lookup into `handlers` map. So there are two possibilities to fix this:
- guard for null result from `singnals` lookup, or
- swap the order of modifying the `signals` map and a call to `handle0` native method. You could even update the `signals` map with `.putIfAbsent()` to avoid multiple reassignment with otherwise equal instances. This may unnecessarily establish a mapping for a Signal the can later not be registered, so you can remove it from the `signals` map in that case to clean-up.
The possible case when the lookup into `handlers` map returns null Handler is already taken into account, but there is a possible case where a NativeHandler is replaced with a java Handler implementation and `dispatch` is therefore already called, but `handlers` map lookup still returns a NativeHandler. In that case the NativeHandler::handle method will throw exception. To avoid that, NativeHandler::handle could be an empty method instead.

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PR: https://git.openjdk.org/jdk/pull/9100