RFR: 8356667: GenShen: Eliminate races with ShenandoahFreeSet::available() [v2]

[William Kemper]

On Mon, 12 May 2025 23:22:33 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Two changes:
>> 
>> 1. Compute the difference between capacity and used while holding the heap lock (so that these two values do not change in incompatible ways while the difference is being computed)
>> 2. Return a "harmless" Sentinel value in case available() is consulted while we are in the process of rebuilding the freeset.  The SIZE_MAX value is considered harmless in that it will not cause immediate trigger of a new GC.  In typical usage, the control or regulator thread will check available() again in another millisecond, at which time a more accurate accounting of available will be provided (if the freeset reconstruction has been completed).
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Respond to reviewer comments

Changes requested by wkemper (Reviewer).

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp line 234:

> 232:   }
> 233: 
> 234:   // Acquire heap lock and return available_in, assuming heap lock is not acquired by the caller.

Sorry - can we change this comment too? This method does _not_ acquire the lock in release builds. Comment could mention that it acquires the lock only for the correctness of the assertion?

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PR Review: https://git.openjdk.org/jdk/pull/25165#pullrequestreview-2834876929
PR Review Comment: https://git.openjdk.org/jdk/pull/25165#discussion_r2085680099