8308043: Deadlock in TestCSLocker.java due to blocking GC while allocating #13989

fisk · 2023-05-15T14:26:42Z

The TestCSLocker.java test spawns a thread that grabs the GC locker, and then wait for the first thread to run some java code and then get signal back to release the GC locker. All of this while another thread is allocating garbage and triggering GCs. Naturally, if the thread that is to signal the release of the GC locker requires GC in order to make progress, we will end up with a deadlock that leads to a timeout. As it turns out, that does indeed happen. A println statement is performed, which in its internal implementation performs an allocation, which requires GC. I think any GC can spuriously fail here, but it seems more likely with generational ZGC for whatever reason. While it seems really shady to wait with the GC locker held while a Java thread executing Java code is supposed to make progress, in general, I think the test can be fixed by removing the println statement causing the allocation. I have run the test 200 times, and it's no longer failing with generational ZGC.

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Issue

JDK-8308043: Deadlock in TestCSLocker.java due to blocking GC while allocating

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Stefan Karlsson (@stefank - Reviewer)
Albert Mingkun Yang (@albertnetymk - Reviewer) ⚠️ Review applies to c4124a0f
Thomas Schatzl (@tschatzl - Reviewer) ⚠️ Review applies to c4124a0f

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bridgekeeper · 2023-05-15T14:27:37Z

👋 Welcome back eosterlund! 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 · 2023-05-15T14:30:15Z

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

hotspot-gc

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mlbridge · 2023-05-15T14:33:46Z

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openjdk · 2023-05-15T14:41:26Z

@fisk 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:

8308043: Deadlock in TestCSLocker.java due to blocking GC while allocating

Reviewed-by: stefank, ayang, tschatzl

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At the time when this comment was updated there had been 22 new commits pushed to the master branch:

8686a36: 8307315: Missing ResourceMark in CDS and JVMTI code
23cbb2d: 8306860: Avoid unnecessary allocation in List.map() when list is empty
be54b54: 8307369: Add execution of all svc tests in CI
19c8c30: 8305634: Renaming predicates, simple cleanups, and adding summary about current predicates
72294c5: 8308024: HttpClient (HTTP/1.1) sends an extraneous empty chunk if the BodyPublisher supplies an empty buffer
c9b6bb5: 8308041: [JVMCI] WB_IsGCSupportedByJVMCICompiler must enter correct JVMCI env
0790f70: 8306806: JMX agent with JDP enabled won't start when PerfData is disabled
c2ef302: 8307968: serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java timed out
d22bcc8: 8307403: java/util/zip/DeInflate.java timed out
2210e06: 8306638: Open source some AWT tests related to datatransfer and Toolkit
... and 12 more: https://git.openjdk.org/jdk/compare/8d49ba9e8d3095f850b3007b56488a0c0cf8ddff...master

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albertnetymk · 2023-05-15T14:48:09Z

test/hotspot/jtreg/gc/cslocker/TestCSLocker.java

@@ -51,7 +51,6 @@ public static void main(String args[]) throws Exception {

        // check timeout to success deadlocking
        while(System.currentTimeMillis() < startTime + timeout) {
-            System.out.println("sleeping...");


I think some comments (one cannot run any gc-triggering code, e.g. println) here would be nice. It's super tempting to add some innocent debug-prints before suspending the current thread, while extending/fixing this test case in the future.

tschatzl · 2023-05-15T15:39:50Z

test/hotspot/jtreg/gc/cslocker/TestCSLocker.java

@@ -51,7 +51,6 @@ public static void main(String args[]) throws Exception {

        // check timeout to success deadlocking
        while(System.currentTimeMillis() < startTime + timeout) {


In addition to @albertnetymk's comment, please also fix the whitespace after the while and the bracket.

fisk · 2023-05-16T11:46:10Z

Thanks for the reviews @stefank @tschatzl and @albertnetymk! I updated with a comment and whitespace fix as suggested, and removed an old problem listing for this test.

fisk · 2023-05-17T11:46:11Z

/integrate

openjdk · 2023-05-17T11:47:35Z

Going to push as commit 285c833.
Since your change was applied there have been 40 commits pushed to the master branch:

1a6f981: 8308185: Update Http2TestServerConnection to use SSLSocket.startHandshake()
b300e73: 8308088: Improve class check in CollectedHeap::is_oop
e34ecc9: 8296469: Instrument VMError::report with reentrant iteration step for register and stack printing
5a92aae: 8308156: VerifyCACerts.java misses blank in error output
2f1c654: 8307955: Prefer to PTRACE_GETREGSET instead of PTRACE_GETREGS in method 'ps_proc.c::process_get_lwp_regs'
d3e5065: 8306460: Clear JVM_ACC_QUEUED flag on methods when dumping dynamic CDS archive
563152f: 8308223: failure handler missed jcmd.vm.info command
cb8b8cd: 8307458: Add periodic heap usage JFR events
f9a785e: 8307547: Support variant collations
4e92991: 8300794: Use @snippet in java.util:i18n
... and 30 more: https://git.openjdk.org/jdk/compare/8d49ba9e8d3095f850b3007b56488a0c0cf8ddff...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-05-17T11:47:48Z

@fisk Pushed as commit 285c833.

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

8308043: Deadlock in TestCSLocker.java due to blocking GC while alloc…

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8308043: Deadlock in TestCSLocker.java due to blocking GC while allocating #13989

8308043: Deadlock in TestCSLocker.java due to blocking GC while allocating #13989

fisk commented May 15, 2023 •

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albertnetymk May 15, 2023

tschatzl May 15, 2023

fisk commented May 16, 2023

fisk commented May 17, 2023

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openjdk bot commented May 17, 2023

		@@ -51,7 +51,6 @@ public static void main(String args[]) throws Exception {

		// check timeout to success deadlocking
		while(System.currentTimeMillis() < startTime + timeout) {

8308043: Deadlock in TestCSLocker.java due to blocking GC while allocating #13989

8308043: Deadlock in TestCSLocker.java due to blocking GC while allocating #13989

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