8271217: Fix race between G1PeriodicGCTask checks and GC request

[kimbarrett]

Please review this change to remove a race involving G1PeriodicGCTask. The
task performs various checks to determine whether a periodic GC should be
performed. If they pass, then it requests the GC by calling try_collect.
The problem is that an unrelated GC could occur after the task's checks but
before its request is processed. In that case we don't want to do the
periodic GC after all, but there's nothing to prevent it.

This change lets the task capture the various GC counters as part of its
checking, and pass them to try_collect for use in detecting an intervening
GC.

Testing:
mach5 tier1

---

Progress

• Change must not contain extraneous whitespace
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Issue

• JDK-8271217: Fix race between G1PeriodicGCTask checks and GC request

Reviewers

• Ivan Walulya (@walulyai - Committer) ⚠️ Review applies to 0152fd5
• Thomas Schatzl (@tschatzl - Reviewer) ⚠️ Review applies to 0152fd5
• Leo Korinth (@lkorinth - Reviewer) ⚠️ Review applies to 0152fd5

Reviewing

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[openjdk]

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[mlbridge]

Webrevs

• 01: Full - Incremental (c48d53b6)
• 00: Full (0152fd58)

[walulyai]

Lgtm!

[openjdk]

@kimbarrett This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8271217: Fix race between G1PeriodicGCTask checks and GC request

Reviewed-by: iwalulya, tschatzl, lkorinth

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[lkorinth]

Looks good to me.

If you like, you could initialize the values of the counters in the default constructor (to zero for example).

[kimbarrett]

If you like, you could initialize the values of the counters in the default constructor (to zero for example).

I think initialized to zero is no better than uninitialized here. Either
way, a default-constructed object of this type shouldn't be given to
try_collect. In an unpublished development version I had a debug-only
is_initialized flag that was set appropriately by the constructors and
checked by the accessors, but decided it more clutter than it was worth and
dropped it.

[kimbarrett]

Thanks @walulyai , @tschatzl , and @lkorinth for reviews.

[kimbarrett]

/integrate

[openjdk]

Going to push as commit 452f7d7.

[openjdk]

@kimbarrett Pushed as commit 452f7d7.

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