> This is a regression from > [JDK-8315884](https://bugs.openjdk.org/browse/JDK-8315884). > > When using `+UseObjectMonitorTable` monitors are inflated in a locked state > effectively blocking out deflation. `LightweightSynchronizer::enter_for` > assumed this to be true. But when the `-UseObjectMonitorTable` path was added > `// Do the old inflate and enter.` this is no longer true as it first > inflates a monitor in an unlocked state and then tries to lock. We need to > introduce a retry loop similar to what was used before > [JDK-8315884](https://bugs.openjdk.org/browse/JDK-8315884). > > I propose we add this retry loop for both cases, to decouple the > `LightweightSynchronizer::enter_for` from how lock elimination is done. With > a retry loop, the only requirements for using > `LightweightSynchronizer::enter_for` is that the Object locked on cannot have > been locked on by another thread, i.e. there is no contention, but makes no > assumptions on the interaction with the deflation thread. > > For `-UseObjectMonitorTable` 7bdbe114eb57fe7310f9664a434c4d9203e656fc should > be enough, as it will assist the deflating thread with deflation, so the > second call must succeed. > > However `+UseObjectMonitorTable` cannot do this so it must wait for the > deflating thread to make progress. But as mentioned above, this would only > happen if partial lock elimination is performed. E.g. > > Object o = new Object(); > synchronized(o) { > o.wait(1); > } > synchronized(o) { > deoptimize(); > } > > got transformed to > > Object o = new Object(); > synchronized(o) { > o.wait(1); > } > // synchronized(o) { Eliminated lock > deoptimize(); > // } > > > As far as I can tell, this does not happen. But I do not want to couple lock > elimination decision with `LightweightSynchronizer::enter_for`. So I propose > a retry loop instead of just the two calls. > After this change the only prerequisite for using > `LightweightSynchronizer::enter_for` is that the object being synchronized > can not have been reached by another JavaThread (except the deflating > thread). So there may never be contention, but there may be deflation.
Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision: Remove @test id ------------- Changes: - all: https://git.openjdk.org/jdk/pull/21420/files - new: https://git.openjdk.org/jdk/pull/21420/files/e16bfde1..bfc5a349 Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=21420&range=01 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21420&range=00-01 Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod Patch: https://git.openjdk.org/jdk/pull/21420.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/21420/head:pull/21420 PR: https://git.openjdk.org/jdk/pull/21420
