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8290867: Race freeing remembered set segments #152

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tschatzl
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@tschatzl tschatzl commented Jul 22, 2022

Hi all,

please review this fix for a crash due to a race in remembered set segment deallocation. Here is the description (provided by chaeubl as reported):

  • Thread A executes G1SegmentedArray::create_new_segment and tries to pop an element from the _free_segment_list. For that, thread A executes LockFreeStack::pop()
  • Thread A reads LockFreeStack::top()
  • Thread B executes LockFreeStack::pop(), also reads LockFreeStack::top() and pops that element from the stack
  • Thread B executes Atomic::cmpxchg(&_first, prev, next); in G1SegmentedArray::create_new_segment but it fails because another thread already registered a different segment
  • Thread B calls G1SegmentedArraySegment::delete_segment and frees the value
  • Thread A tries to access top()->next in LockFreeStack::pop(), which causes a segfault because top() was freed by thread B

The fix is to delay the deletion of that memory segment until all readers (i.e. in G1SegmentedArrayFreeList::get calling _list.pop()) drop the references to that memory segment. The readers are already guarded by a CriticalSection.

Testing: tier1-5 running, reproducer that adds extra delays that significantly delays to widen the opportunity this race can occur passes on BigRAMTester (otherwise crashes in a few seconds)

Thanks,
Thomas


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bridgekeeper bot commented Jul 22, 2022

👋 Welcome back tschatzl! 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 openjdk bot changed the title 8290867 8290867: Race freeing remembered set segments Jul 22, 2022
@openjdk openjdk bot added the rfr Pull request is ready for review label Jul 22, 2022
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openjdk bot commented Jul 22, 2022

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

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@openjdk openjdk bot added the hotspot-gc hotspot-gc-dev@openjdk.org label Jul 22, 2022
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mlbridge bot commented Jul 22, 2022

Webrevs

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Looks good.

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openjdk bot commented Jul 22, 2022

@tschatzl 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:

8290867: Race freeing remembered set segments

Reviewed-by: kbarrett, sangheki

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 5 new commits pushed to the master branch:

  • 54c093a: 8291524: jdk/jfr/event/runtime/TestClassLoaderStatsEvent.java Value not equal to 2, field='hiddenClassCount', value='0'
  • 1d16c91: 8291512: Snippetize modules API examples
  • 36c00fd: 8291006: java/foreign/TestUnsupportedPlatform fails after JDK-8290455
  • 8c9d5ad: 8290455: jck test api/java_lang/foreign/VaList/Empty.html fails on some platforms
  • c29242e: 8290460: Alpine: disable some panama tests that rely on std::thread

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Looks good.

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tschatzl commented Aug 3, 2022

Testing showed an issue with VM shutdown: VM shutdown calls the the destructor for the (global) free list pool, which in turn frees all segments; however at that time the VM is not in a state where GlobalCounter works (threads are detached, no valid threads list) and crashes.
The last commit provides a workaround for that issue, not doing the global synchronization when shutting down. That is safe because the process is single-threaded in the VM_Exit operation anyway.
This is a workaround for this issue that needs to be cleaned up, but it's too late imho in the release to find a better solution for this issue.

Passes tier1-5 with no crashes.

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tschatzl commented Aug 3, 2022

Thanks @kimbarrett @sangheon for your reviews
/integrate

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openjdk bot commented Aug 3, 2022

Going to push as commit e265b2a.
Since your change was applied there have been 5 commits pushed to the master branch:

  • 54c093a: 8291524: jdk/jfr/event/runtime/TestClassLoaderStatsEvent.java Value not equal to 2, field='hiddenClassCount', value='0'
  • 1d16c91: 8291512: Snippetize modules API examples
  • 36c00fd: 8291006: java/foreign/TestUnsupportedPlatform fails after JDK-8290455
  • 8c9d5ad: 8290455: jck test api/java_lang/foreign/VaList/Empty.html fails on some platforms
  • c29242e: 8290460: Alpine: disable some panama tests that rely on std::thread

Your commit was automatically rebased without conflicts.

@openjdk openjdk bot added the integrated Pull request has been integrated label Aug 3, 2022
@openjdk openjdk bot closed this Aug 3, 2022
@openjdk openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Aug 3, 2022
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openjdk bot commented Aug 3, 2022

@tschatzl Pushed as commit e265b2a.

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

@tschatzl tschatzl deleted the submit/8290867-free-remembered-sets branch August 3, 2022 16:15
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Testing showed an issue with VM shutdown: VM shutdown calls the the destructor for the (global) free list pool, which in turn frees all segments; however at that time the VM is not in a state where GlobalCounter works (threads are detached, no valid threads list) and crashes. The last commit provides a workaround for that issue, not doing the global synchronization when shutting down. That is safe because the process is single-threaded in the VM_Exit operation anyway. This is a workaround for this issue that needs to be cleaned up, but it's too late imho in the release to find a better solution for this issue.

For the record, we discussed this before Thomas pushed, and I agree with his assessment. The workaround is simple and safe, and doing something more principled might not fit in the current release schedule.

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