8299673: Simplify object pinning interactions with string deduplication #11923

fisk · 2023-01-10T10:04:48Z

When raw char* String contents are exposed to JNI code, we

Load the string.value and pin it
Run native code
Load the string.value and unpin it

Given this sequence we would be in trouble if between 1 and 3, string deduplication changed the value object. Then the pinning and unpinning wouldn't be balanced.

The current approach for dealing with this is to have a bunch of code to guard against deduplication. An alternative simpler solution is to just change step 3 to pass in the same value. We already have enough information available to do that. Then the pinning and unpinning is also balanced, and we don't need to have any special interactions with string deduplication and can decouple these orthogonal concerns.

It's worth noting though that the contract of pin_object now makes it explicit that pinned objects must not be recycled, even if not otherwise reachable. That seems to come naturally for region based pinning, but is worth keeping in mind. The exposed char* might be the only thing referencing the string value when string dedup happens concurrently.

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Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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Commit message must refer to an issue

Issue

JDK-8299673: Simplify object pinning interactions with string deduplication

Reviewers

Kim Barrett (@kimbarrett - Reviewer) ⚠️ Review applies to 3e1eb94d
Stefan Karlsson (@stefank - Reviewer) ⚠️ Review applies to 3e1eb94d
David Holmes (@dholmes-ora - Reviewer)

Contributors

Stefan Karlsson <stefank@openjdk.org>
Axel Boldt-Christmas <aboldtch@openjdk.org>

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk pull/11923/head:pull/11923
$ git checkout pull/11923

Update a local copy of the PR:
$ git checkout pull/11923
$ git pull https://git.openjdk.org/jdk pull/11923/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 11923

View PR using the GUI difftool:
$ git pr show -t 11923

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/11923.diff

bridgekeeper · 2023-01-10T10:06:08Z

👋 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 · 2023-01-10T10:09:01Z

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

hotspot
shenandoah

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

mlbridge · 2023-01-10T10:12:50Z

Webrevs

01: Full (3c6ab636)
00: Full (3e1eb94d)

fisk · 2023-01-10T10:22:25Z

/contributor add @stefank

openjdk · 2023-01-10T10:24:06Z

@fisk
Contributor Stefan Karlsson <stefank@openjdk.org> successfully added.

fisk · 2023-01-10T10:24:26Z

/contributor add @xmas92

openjdk · 2023-01-10T10:25:49Z

@fisk
Contributor Axel Boldt-Christmas <aboldtch@openjdk.org> successfully added.

kimbarrett

Looks good.

openjdk · 2023-01-11T04:45:41Z

@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:

8299673: Simplify object pinning interactions with string deduplication

Co-authored-by: Stefan Karlsson <stefank@openjdk.org>
Co-authored-by: Axel Boldt-Christmas <aboldtch@openjdk.org>
Reviewed-by: kbarrett, stefank, dholmes

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 7 new commits pushed to the master branch:

f52f6e6: 8299972: Remove metaprogramming/removeReference.hpp
4c1e66e: 8299971: Remove metaprogramming/conditional.hpp
4073b68: 8298047: Remove all non-significant trailing whitespace from properties files
506c818: 8296401: ConcurrentHashTable::bulk_delete might miss to delete some objects
319de6a: 8300124: Remove unnecessary assert in GenCollectedHeap::initialize
289aed4: 8298128: runtime/ErrorHandling/TestSigInfoInHsErrFile.java fails to find pattern with slowdebug
a2f6766: 8288415: java/awt/PopupMenu/PopupMenuLocation.java is unstable in MacOS machines

Please see this link for an up-to-date comparison between the source branch of this pull request and the master branch.
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.

stefank · 2023-01-11T08:33:03Z

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

+#include "gc/shared/gcLocker.inline.hpp"
 #include "gc/shared/gcHeapSummary.hpp"


fisk · 2023-01-13T16:19:05Z

Thank you for the reviews, @kimbarrett and @stefank!

dholmes-ora

Initially I was a bit unsure about the conceptual model here, as I was thinking that pinning is a very general concept, where in fact it only relates to these JNI "critical" functions. So in that sense every GC must support pinning as required by those functions, so this simplification looks very neat. Thanks.

fisk · 2023-01-17T07:55:36Z

Thanks for the review, @dholmes-ora!

fisk · 2023-01-17T07:59:26Z

/integrate

openjdk · 2023-01-17T08:01:13Z

Going to push as commit 9a36f8a.
Since your change was applied there have been 15 commits pushed to the master branch:

3462438: 8299576: Reimplement java.io.Bits using VarHandle access
f829a67: 8300227: [macos_aarch64] assert(cpu_has("hw.optional.arm.FEAT_AES")) failed after JDK-8297092
6a81d52: 8299213: Bad cast in GrowableArrayWithAllocator<>::grow
8365c67: 8300084: AquaFileChooserUI.getDefaultButton returns null
382fe51: 8163229: several regression tests have a main method that is never executed
06f9374: 8298867: Basics.java fails with SSL handshake exception
859ccd1: 8299847: RISC-V: Improve PrintOptoAssembly output of CMoveI/L nodes
240830d: 8297092: [macos_aarch64] Add support for SHA feature detection
f52f6e6: 8299972: Remove metaprogramming/removeReference.hpp
4c1e66e: 8299971: Remove metaprogramming/conditional.hpp
... and 5 more: https://git.openjdk.org/jdk/compare/a734285314a34ed61583132f2fc6be9d9c861af4...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-01-17T08:01:32Z

@fisk Pushed as commit 9a36f8a.

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

fisk added 3 commits January 9, 2023 09:48

8299673: Simplify object pinning interactions with string deduplication

bc4215f

Feedback from Kim

be175da

More Kim feedback

3e1eb94

openjdk bot added the rfr Pull request is ready for review label Jan 10, 2023

openjdk bot added hotspot hotspot-dev@openjdk.org shenandoah shenandoah-dev@openjdk.org labels Jan 10, 2023

kimbarrett approved these changes Jan 11, 2023

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openjdk bot added the ready Pull request is ready to be integrated label Jan 11, 2023

stefank approved these changes Jan 11, 2023

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fisk added 2 commits January 16, 2023 11:12

Merge branch 'master' into 8299673_pin_dedup

f22a278

Include sorting order

3c6ab63

dholmes-ora approved these changes Jan 17, 2023

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openjdk bot added the integrated Pull request has been integrated label Jan 17, 2023

openjdk bot closed this Jan 17, 2023

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Jan 17, 2023

		#include "gc/shared/gcLocker.inline.hpp"
		#include "gc/shared/gcHeapSummary.hpp"

8299673: Simplify object pinning interactions with string deduplication #11923

8299673: Simplify object pinning interactions with string deduplication #11923

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