8275049: [ZGC] missing null check in ZNMethod::log_register #5892

TheRealMDoerr · 2021-10-11T12:23:03Z

The VM crashes while trying to read (*p)->klass() in "ZNMethod::log_register" on PPC64. We need a null check. See JBS for details.

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Issue

JDK-8275049: [ZGC] missing null check in ZNMethod::log_register

Reviewers

Niklas Radomski (@Quaffel - Author) ⚠️ Review applies to bf1a79e
Erik Österlund (@fisk - Reviewer)
Per Liden (@pliden - Reviewer)

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bridgekeeper · 2021-10-11T12:23:43Z

👋 Welcome back mdoerr! 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 · 2021-10-11T12:25:19Z

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

hotspot-gc

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mlbridge · 2021-10-11T12:30:22Z

Webrevs

fisk

The oops we read here are written in c1_Runtime, while holding the Patching_lock. However, the Patching_lock is not held when registering. In other words, someone could be writing an oop while we are reading it in this loop, due to C1 patching. If the loads re-order in here, we may crash the VM. Since we have plain loads, the compiler is free to re-order. I think the solution I would go with, is to use the CompiledICLocker instead where we patch the code in C1. That ends up taking the per-nmethod lock, that we can hold while logging this. That makes sure that accessing the oops implies mutual exclusion. Then we can remove the Patching_lock, since it is only ever used in that one place.

TheRealMDoerr · 2021-10-12T13:52:01Z

Hi Erik,
your description sounds like the plain reads are what are problematic. (Oop creation always needs to release the header initialization. Otherwise, it's a bug there.) I agree with that. I could use Atomic::load(p) or Atomic::load_acquire(p). What do you think about that?
Fundamental locking changes are out of scope, here. That could be done separately if desired. This issue is about the missing null check. Note that we may need to backport this fix, so we should better keep it simple.

fisk · 2021-10-12T14:17:21Z

I think we could just Atomic::load(p) for now to deal with the null check, and then see if we want to deal with the other race condition another day.

pliden · 2021-10-12T14:34:24Z

src/hotspot/share/gc/z/zNMethod.cpp

@@ -126,8 +126,10 @@ void ZNMethod::log_register(const nmethod* nm) {
    oop* const begin = nm->oops_begin();
    oop* const end = nm->oops_end();
    for (oop* p = begin; p < end; p++) {
+      const char* external_name = (*p) == nullptr ? "null"
+                                                  : (*p)->klass()->external_name();


Apart from doing an Atomic::load() here, I'd also suggest we write "N/A" instead of "null", since the class isn't really null.

… of null oop.

TheRealMDoerr · 2021-10-12T15:18:10Z

Thanks for your suggestions. They make sense.

fisk

Looks good.

openjdk · 2021-10-12T15:32:29Z

@TheRealMDoerr 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:

8275049: [ZGC] missing null check in ZNMethod::log_register

Reviewed-by: nradomski, eosterlund, pliden

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 27 new commits pushed to the master branch:

124f823: 8268764: Use Long.hashCode() instead of int-cast where applicable
8657f77: 8271514: support JFR use of new ThreadsList::Iterator
b8bd259: 8271737: Only normalize the cached user.dir property once
89999f7: 8275131: Exceptions after a touchpad gesture on macOS
07b1f1c: 8274548: (fc) FileChannel gathering write fails with IOException "Invalid argument" on macOS 11.6
f623460: 8274911: testlibrary_tests/ir_framework/tests/TestIRMatching.java fails with "java.lang.RuntimeException: Should have thrown exception"
e393c5e: 8275074: Cleanup unused code in JFR LeakProfiler
e16b93a: 8274770: [PPC64] resolve_jobject needs a generic implementation to support load barriers
1ab6414: 8275051: Shenandoah: Correct ordering of requested gc cause and gc request flag
b460d6d: 8275091: /src/jdk.management.jfr/share/classes/module-info.java has non-canonical order
... and 17 more: https://git.openjdk.java.net/jdk/compare/b7af890574b3c13122fe7de987a8c9458c05f625...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.

pliden · 2021-10-12T16:16:37Z

src/hotspot/share/gc/z/zNMethod.cpp

      log_oops.print("           Oop[" SIZE_FORMAT "] " PTR_FORMAT " (%s)",
-                     (p - begin), p2i(*p), (*p)->klass()->external_name());
+                     (p - begin), p2i(*p), external_name);


p2i(*p) should be p2i(o) now

pliden · 2021-10-12T16:17:30Z

src/hotspot/share/gc/z/zNMethod.cpp

@@ -126,8 +126,11 @@ void ZNMethod::log_register(const nmethod* nm) {
    oop* const begin = nm->oops_begin();
    oop* const end = nm->oops_end();
    for (oop* p = begin; p < end; p++) {
+      oop o = Atomic::load(p); // C1 PatchingStub may replace it concurrently.
+      const char* external_name = o == nullptr ? "N/A"
+                                               : o->klass()->external_name();


Can we please make this a single line, and add parenthesis around (o == nullptr)

pliden · 2021-10-12T16:17:48Z

src/hotspot/share/gc/z/zNMethod.cpp

@@ -126,8 +126,11 @@ void ZNMethod::log_register(const nmethod* nm) {
    oop* const begin = nm->oops_begin();
    oop* const end = nm->oops_end();
    for (oop* p = begin; p < end; p++) {
+      oop o = Atomic::load(p); // C1 PatchingStub may replace it concurrently.


Can we please make this const oop o = ...

TheRealMDoerr · 2021-10-12T16:49:26Z

Good catch. Thanks!

pliden

Thanks for fixing. Looks good!

TheRealMDoerr · 2021-10-13T07:11:25Z

Thanks for the reviews!
/integrate

openjdk · 2021-10-13T07:13:00Z

Going to push as commit cf82867.
Since your change was applied there have been 30 commits pushed to the master branch:

ab34cce: 8275186: Suppress warnings on non-serializable array component types in xml
b1b8350: 8275171: ProblemList compiler/codegen/aes/TestAESMain.java on linux-x64 and windows-x64 in -Xcomp mode
03c2b73: 8275128: Build hsdis using normal build system
124f823: 8268764: Use Long.hashCode() instead of int-cast where applicable
8657f77: 8271514: support JFR use of new ThreadsList::Iterator
b8bd259: 8271737: Only normalize the cached user.dir property once
89999f7: 8275131: Exceptions after a touchpad gesture on macOS
07b1f1c: 8274548: (fc) FileChannel gathering write fails with IOException "Invalid argument" on macOS 11.6
f623460: 8274911: testlibrary_tests/ir_framework/tests/TestIRMatching.java fails with "java.lang.RuntimeException: Should have thrown exception"
e393c5e: 8275074: Cleanup unused code in JFR LeakProfiler
... and 20 more: https://git.openjdk.java.net/jdk/compare/b7af890574b3c13122fe7de987a8c9458c05f625...master

Your commit was automatically rebased without conflicts.

openjdk · 2021-10-13T07:13:22Z

@TheRealMDoerr Pushed as commit cf82867.

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

8275049: [ZGC] missing null check in ZNMethod::log_register

bf1a79e

openjdk bot added the rfr Pull request is ready for review label Oct 11, 2021

openjdk bot added the hotspot-gc hotspot-gc-dev@openjdk.org label Oct 11, 2021

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fisk suggested changes Oct 12, 2021

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openjdk bot mentioned this pull request Oct 12, 2021

8274851: [PPC64] Port zgc to linux on ppc64le #5842

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pliden reviewed Oct 12, 2021

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Use Atomic::load to read the oop and change klass name to N/A in case…

5f48b72

… of null oop.

fisk approved these changes Oct 12, 2021

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openjdk bot added the ready Pull request is ready to be integrated label Oct 12, 2021

pliden suggested changes Oct 12, 2021

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Use o instead of *p plus further cleanup.

f6fe2e4

pliden approved these changes Oct 12, 2021

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fisk approved these changes Oct 12, 2021

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openjdk bot closed this Oct 13, 2021

openjdk bot added integrated Pull request has been integrated and removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Oct 13, 2021

TheRealMDoerr deleted the 8275049_ZGC_log_register branch October 13, 2021 07:22

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8275049: [ZGC] missing null check in ZNMethod::log_register #5892

8275049: [ZGC] missing null check in ZNMethod::log_register #5892

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8275049: [ZGC] missing null check in ZNMethod::log_register #5892

8275049: [ZGC] missing null check in ZNMethod::log_register #5892

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