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8275054: Racy updates when releasing confined scopes #596

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mcimadamore
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@mcimadamore mcimadamore commented Oct 11, 2021

As the subject says, there are cases where a close dependency on a confined parent scope can occur from an unrelated thread:

  1. the confined scope is acquired by another confined scope featuring implicit cleanup - in which case the cleaner thread will do the release

  2. the confined scope is acquired, in the same thread, by a shared scope. Since a shared scope can be closed by any other thread, it follows that release can also be done by any thread.

In such cases we can have races (either between release/release or between acquire/release) and the lockCount variable of the updated scope becomes garbage.

The solution is to add an extra variable asyncReleaseCount. If a release request occur from the same thread that owns the confined scope, we proceed as usual, otherwise we atomically increment the asyncReleaseCount. When the confined scope is later closed, if lockCount == 0, nothing needs to be done, so we can just close. Otherwise, we need to check that lockCount - asyncReleaseCount == 0. This works because a confined segment can only be acquired by the same thread owning it. So, if lockCount == 0 there can be no other pending release.


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  • JDK-8275054: Racy updates when releasing confined scopes

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@mcimadamore mcimadamore commented Oct 11, 2021

Another solution would have been to trigger an exception when setting up a dependency from a shared scope to a confined scope - but we still need the same logic for implicitly closeable scopes anyway.

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@bridgekeeper bridgekeeper bot commented Oct 11, 2021

👋 Welcome back mcimadamore! A progress list of the required criteria for merging this PR into foreign-memaccess+abi 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 added the rfr label Oct 11, 2021
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@mlbridge mlbridge bot commented Oct 11, 2021

Webrevs

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@openjdk openjdk bot commented Oct 11, 2021

@mcimadamore 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:

8275054: Racy updates when releasing confined scopes

Reviewed-by: jvernee

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At the time when this comment was updated there had been 1 new commit pushed to the foreign-memaccess+abi branch:

  • 9608462: 8275061: Exceptions thrown from non-void upcalls are not handled

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@openjdk openjdk bot added the ready label Oct 11, 2021
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@mcimadamore mcimadamore commented Oct 11, 2021

/integrate

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@openjdk openjdk bot commented Oct 11, 2021

Going to push as commit b0cf974.
Since your change was applied there has been 1 commit pushed to the foreign-memaccess+abi branch:

  • 9608462: 8275061: Exceptions thrown from non-void upcalls are not handled

Your commit was automatically rebased without conflicts.

@openjdk openjdk bot closed this Oct 11, 2021
@openjdk openjdk bot added integrated and removed ready rfr labels Oct 11, 2021
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@openjdk openjdk bot commented Oct 11, 2021

@mcimadamore Pushed as commit b0cf974.

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

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