On Thu, 26 Nov 2020 21:23:16 GMT, Vladimir Ivanov <vliva...@openjdk.org> wrote:
> Concurrent updates may lead to redundant LambdaForms created and unnecessary > class loading when those are compiled. > > Most notably, it severely affects MethodHandle customization: when a > MethodHandle is called from multiple threads, every thread starts > customization which takes enough time for other threads to join, but only one > of those customizations will be picked. > > Coordination between threads requesting the updates and letting a single > thread proceed avoids the aforementioned problem. Moreover, there's no need > to wait until the update in-flight is over: all other threads (except the one > performing the update) can just proceed with the invocation using the > existing MH.form. > > Testing: > - manually monitored the behavior on a stress test from > [JDK-8252049](https://bugs.openjdk.java.net/browse/JDK-8252049) > - tier1-4 src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 1783: > 1781: } finally { > 1782: updateInProgress = false; > 1783: } Line 1782. I understand that in case the try block fails to update `form` field, resetting `updateInProgress` is desirable in order for next caller to attempt to update the form next time, but in case (the majority case) the try block successfully updates the `form` field, resetting the `updateInProgress` back to `false` opens a theoretical window (very improbable) for a concurrent thread to re-execute the updater function at least. Perhaps you could write `catch (Throwable e)` instead of `finally` and only in case of failure, reset the `updateInProgress` field and re-throw the exception. WDYT? It is also very strange that the code is updating a final `form` field using Unsafe. Is this only to ensure that reads of that field scattered around in code are accompanied with a suitable read fence if needed (on some architectures)? How does this work considering (I may be wrong, but I think I heard) that VM trusts final fields in java.lang.invoke package. ------------- PR: https://git.openjdk.java.net/jdk/pull/1472
