8295375: debug agent class tracking should not piggy back on the cbClassPrepare() callback #10776

plummercj · 2022-10-20T01:21:19Z

The debug agent needs to keep track of all loaded classes, and also be notified when they are unloaded. It tracks classes loading by getting CLASS_PREPARE events and it tracks their unloading by tagging them, which triggers OBJECT_FREE events when they are unloaded. The tagging and OBJECT_FREE events are handled by a separate JVMTIEnv from the one that does main event handling, mostly in support of the attached debugger. However, the CLASS_PREPARE events are piggy backed on the main event handler. As a result, we have this special check in filterAndHandleEvent():

    /* We must keep track of all classes prepared to know what's unloaded */
    if (evinfo->ei == EI_CLASS_PREPARE) {
        classTrack_addPreparedClass(env, evinfo->clazz);
    }

We also have to always keep CLASS_PREPARE events enabled on the main event handler, even if the debugger is not requesting them. The main event handler has a lot of overhead that isn't necessary when simply wanting to use the CLASS_PREPARE event for class tracking.

Another downside of this piggy backing is it causes problems for addressing JDK-8295376, which is attempting to not track virtual threads when the debugger is not attached (no VIRTUAL_THREAD_START and VIRTUAL_THREAD_END events). The problem is when the debugger is not attached and a CLASS_PREPARE event comes in on a virtual thread, the debug agent unnecessarily creates a ThreadNode for it. Since there won't be a corresponding VIRTUAL_THREAD_END event when the virtual thread is destroyed (as long as the debugger is not attached), the debug agent ends up keeping this ThreadNode around even after the thread is gone. This usually eventually leads to an assert.

The fix for this is pretty simple. We already have the separate JVMTIEnv that the class tracker uses to handle OBJECT_FREE. This is easily purposed to also handle CLASS_PREPARE events. By doing so we can get rid of the special CLASS_PREPARE code above in filterAndHandleEvent(), and we also only need to enable CLASS_PREPARE events for the main event handler when a debugger is attached and is requesting them.

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Issue

JDK-8295375: debug agent class tracking should not piggy back on the cbClassPrepare() callback

Reviewers

Alex Menkov (@alexmenkov - Reviewer)
Serguei Spitsyn (@sspitsyn - Reviewer)

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…e() callback

plummercj · 2022-10-20T01:24:02Z

src/jdk.jdwp.agent/share/native/libjdwp/eventFilter.c

@@ -1265,7 +1265,6 @@ enableEvents(HandlerNode *node)
        case EI_THREAD_END:
        case EI_VM_INIT:
        case EI_VM_DEATH:
-        case EI_CLASS_PREPARE:


Since CLASS_PREPARE is no longer globally and permanently enabled, we remove it from this list so the code below can enable it as needed based on whether or not the debugger as requested CLASS_PREPARE events.

plummercj · 2022-10-20T01:25:05Z

src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c

-        /* We must keep track of all classes prepared to know what's unloaded */
-        if (evinfo->ei == EI_CLASS_PREPARE) {
-            classTrack_addPreparedClass(env, evinfo->clazz);
-        }
-


We no longer need this hack here since the class tracking code now installs its own CLASS_PREPARE event handler.

plummercj · 2022-10-20T01:26:20Z

src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c

-    error = threadControl_setEventMode(JVMTI_ENABLE,
-                                       EI_CLASS_PREPARE, NULL);
-    if (error != JVMTI_ERROR_NONE) {
-        EXIT_ERROR(error,"Can't enable class prepare events");
-    }


We no longer permanently enable CLASS_PREPARE events here. It's now done in classTrack.c, and only impacts the JVMTIEnv that is used for class tracking.

bridgekeeper · 2022-10-20T01:26:25Z

👋 Welcome back cjplummer! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2022-10-20T01:29:35Z

@plummercj The following label will be automatically applied to this pull request:

serviceability

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2022-10-20T03:19:40Z

Webrevs

00: Full (e4d35c84)

openjdk · 2022-10-20T19:24:28Z

@plummercj This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8295375: debug agent class tracking should not piggy back on the cbClassPrepare() callback

Reviewed-by: amenkov, sspitsyn

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 21 new commits pushed to the master branch:

ef62b61: 8295703: RISC-V: Remove implicit noreg temp register arguments in MacroAssembler
6240431: 8295697: Resolve conflicts between serviceability/jvmti and nsk/jvmti shared code
1164258: 8295124: Atomic::add to pointer type may return wrong value
d3eba85: 8295414: [Aarch64] C2: assert(false) failed: bad AD file
028e8b3: 8137022: Concurrent refinement thread adjustment and (de-)activation suboptimal
faa6b66: 8295715: Minimize disabled warnings in serviceability libs
de1e0c5: 8295719: Remove unneeded disabled warnings in jdk.sctp
9612cf9: 8295529: Add link to JBS to README.md
b37421e: 8295564: Norwegian Nynorsk Locale is missing formatting
6707bfb: 8029633: Raw inner class constructor ref should not perform diamond inference
... and 11 more: https://git.openjdk.org/jdk/compare/8d4c077218748d37617fc1bdb537a165706a5849...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

sspitsyn

Good simplification.
Thanks,
Serguei

plummercj · 2022-10-21T18:33:18Z

Thank you Alex and Serguei!

/integrate

openjdk · 2022-10-21T18:34:57Z

Going to push as commit 2181042.
Since your change was applied there have been 29 commits pushed to the master branch:

f41711e: 8295650: JFR: jfr scrub should warn if an event type doesn't exist
0c13d66: 8295530: Update Zlib Data Compression Library to Version 1.2.13
15bebf9: 8295666: Linux x86 build fails after 8292591
5064718: 8294460: CodeSection::alignment checks for CodeBuffer::SECT_STUBS incorrectly
8b010e0: 8030616: sun/management/jmxremote/bootstrap/RmiBootstrapTest fails intermittently with cannot find a free port
b35922b: 8295714: GHA ::set-output is deprecated and will be removed
dfd2d83: 8295657: SA: Allow larger object alignments
a345df2: 8280131: jcmd reports "Module jdk.jfr not found." when "jdk.management.jfr" is missing
ef62b61: 8295703: RISC-V: Remove implicit noreg temp register arguments in MacroAssembler
6240431: 8295697: Resolve conflicts between serviceability/jvmti and nsk/jvmti shared code
... and 19 more: https://git.openjdk.org/jdk/compare/8d4c077218748d37617fc1bdb537a165706a5849...master

Your commit was automatically rebased without conflicts.

openjdk · 2022-10-21T18:35:20Z

@plummercj Pushed as commit 2181042.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

debug agent class tracking should not piggy back on the cbClassPrepar…

e4d35c8

…e() callback

plummercj commented Oct 20, 2022

View reviewed changes

openjdk bot changed the title ~~8295375~~ 8295375: debug agent class tracking should not piggy back on the cbClassPrepare() callback Oct 20, 2022

openjdk bot added the serviceability serviceability-dev@openjdk.org label Oct 20, 2022

plummercj marked this pull request as ready for review October 20, 2022 03:13

openjdk bot added the rfr Pull request is ready for review label Oct 20, 2022

alexmenkov approved these changes Oct 20, 2022

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sspitsyn approved these changes Oct 21, 2022

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8295375: debug agent class tracking should not piggy back on the cbClassPrepare() callback #10776

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8295375: debug agent class tracking should not piggy back on the cbClassPrepare() callback #10776

8295375: debug agent class tracking should not piggy back on the cbClassPrepare() callback #10776

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