Re: RFC: 8229160: Reimplement JvmtiRawMonitor to use PlatformMonitor

[serguei.spit...@oracle.com]

Hi David, On 9/8/19 19:15, David Holmes wrote: Hi Dan, On 7/09/2019 6:50 am, Daniel D. Daugherty wrote: Hi David, I've finally gotten back to this email thread... Thanks. FYI testing to date:  - tiers 1 -3 all platforms  - hotspot: serviceability/jvmti                           /jdwp             vmTestbase/nsk/jvmti                           /jdwp  - JDK: com/sun/jdi You should also add: open/test/hotspot/jtreg/vmTestbase/nsk/jdb open/test/hotspot/jtreg/vmTestbase/nsk/jdi open/test/jdk/java/lang/instrument Okay - in progress. Though I can't see any use of RawMonitors in any of these tests. I took a quick look through the preliminary webrev and I don't see anything that worries me. Thanks. I'll prepare a more polished webrev soon. Re: Thread.interrupt() and raw_wait() It would be good to see if that semantic is being tested via the JCK test suite for JVM/TI. It isn't. The only thing directly tested for RawMonitorWait is normal successful operation and reporting "not owner" when not the owner. No check for JVMTI_ERROR_INTERRUPT exists other than as input for the GetErrorName function. This is most likely true. My only concern is if RawMonitor's can be used in the JCK test libraries (low probability). I've asked Leonid Kuskov (JCK) to double check this (added to the mailing list). There's only one test in the whole test base that checks for the interrupt and that is vmTestbase/nsk/jvmti/RawMonitorWait/rawmnwait005/. In that test if we are not interrupted before the RawMonitorWait we will wait until the full timeout elapses - which is 2 minutes by default - then return and report the interrupt. Hence the test still passes. (If it was an untimed wait that would be different of course). I figured the same last Friday. One more place to care about are NSK tests libraries that are located here:   test/hotspot/jtreg/vmTestbase/nsk/share There are a couple of places where the RawMonitor is used:   jvmti/hotswap/HotSwap.cpp:    if (!NSK_JVMTI_VERIFY(jvmti->RawMonitorWait(waitLock, millis)))   jvmti/jvmti_tools.cpp:    jvmtiError error = env->RawMonitorWait(monitor, millis); The use in HotSwap.cpp is local. The jvmti_tools.cpp defines this:   void rawMonitorWait(jvmtiEnv *env, jrawMonitorID monitor, jlong millis) {       jvmtiError error = env->RawMonitorWait(monitor, millis);       exitOnError(error);   } which is used in the jvmti/agent_tools.cpp but does not depend on interrupting of RawMonitor's (as I can see). One more place to mention is:   jvmti/DataDumpRequest/datadumpreq001/datadumpreq001.cpp But I see no problems there as well. The JDWP implementation is using RawMonitor's. Please, see functions debugMonitorWait()/debugMonitorTimedWait() in src/jdk.jdwp.agent/share/native/libjdwp/util.c. It expects the JVMTI_ERROR_INTERRUPT but never makes a call to the JVMTI ThreadInterrupt(). So, it looks like it does not depend on interrupting of RawMonitor's in any way. The more I try to convince people this change should be okay, the more uncomfortable I get with my own arguments. :) I think I'm going to implement the polling approach for checking interrupts - say 500ms. The JVMTI spec tells that the JVMTI_ERROR_INTERRUPT can be returned from the RawMonitorWait:   https://docs.oracle.com/en/java/javase/11/docs/specs/jvmti.html#RawMonitorWait which means that RawMonitorWait can be interrupted with the Thread.Interrupt() or JVMTI InterruptThread():   https://docs.oracle.com/en/java/javase/11/docs/specs/jvmti.html#InterruptThread Thanks, Serguei I also very much like/appreciate the decoupling of JvmtiRawMonitors from ObjectMonitors... Thanks for tackling this crazy task. Thanks :) David Dan On 8/15/19 2:22 AM, David Holmes wrote: Bug: https://bugs.openjdk.java.net/browse/JDK-8229160 Preliminary webrev (still has rough edges): http://cr.openjdk.java.net/~dholmes/8229160/webrev.prelim/ Background: We've had this comment for a long time:  // The raw monitor subsystem is entirely distinct from normal  // java-synchronization or jni-synchronization.  raw monitors are not  // associated with objects.  They can be implemented in any manner  // that makes sense.  The original implementors decided to piggy-back  // the raw-monitor implementation on the existing Java objectMonitor mechanism.  // This flaw needs to fixed.  We should reimplement raw monitors as sui-generis.  // Specifically, we should not implement raw monitors via java monitors.  // Time permitting, we should disentangle and deconvolve the two implementations  // and move the resulting raw monitor implementation over to the JVMTI directories.  // Ideally, the raw monitor implementation would be built on top of  // park-unpark and nothing else. This is an attempt to do that disentangling so that we can then consider changes to ObjectMonitor without having to worry about JvmtiRawMonitors. But rather than building on low-level park/unpark (which would require the same manual queue management and much of the same complex code as exists in ObjectMonitor) I decided to try and do this on top of PlatformMonitor. The reason this is just a RFC rather than RFR is that I overlooked a non-trivial aspect of JvmtiRawMonitors: like Java monitors (as implemented by ObjectMonitor) they interact with the Thread.interrupt mechanism. This is not clearly stated in the JVM TI specification [1] but only in passing by the possible errors for RawMonitorWait: JVMTI_ERROR_INTERRUPT    Wait was interrupted, try again As I explain in the bug report there is no way to build in proper interrupt support using PlatformMonitor as there is no way we can "interrupt" the low-level pthread_cond_wait. But we can approximate it. What I've done in this preliminary version is just check interrupt state before and after the actual "wait" but we won't get woken by the interrupt once we have actually blocked. Alternatively we could use a periodic polling approach and wakeup every Nms to check for interruption. The only use of JvmtiRawMonitors in the JDK libraries (JDWP) is not affected by this choice as that code ignores the interrupt until the real action it was waiting for has occurred. The interrupt is then reposted later. But more generally there could be users of JvmtiRawMonitors that expect/require that RawMonitorWait is responsive to Thread.interrupt in a manner similar to Object.wait. And if any of them are reading this then I'd like to know - hence this RFC :) FYI testing to date:  - tiers 1 -3 all platforms  - hotspot: serviceability/jvmti                           /jdwp             vmTestbase/nsk/jvmti                           /jdwp  - JDK: com/sun/jdi Comments/opinions appreciated. Thanks, David [1] https://docs.oracle.com/en/java/javase/11/docs/specs/jvmti.html#RawMonitorWait