RFR: 8257641: Shenandoah: Query is_at_shenandoah_safepoint() from control thread should return false

[Aleksey Shipilev]

On Thu, 3 Dec 2020 15:05:33 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Since Shenandoah GC safepoints are scheduled by control thread, so that, if querying comes from control thread, the answer should be false.
> 
> is_at_shenandoah_safepoint() is still not reliable, even after JDK-8253778, we may consider to scratch it.
> 
> - [x] hotspot_gc_shenandoah x86_64 and x86_32

This looks good, consider fixing a few nits below.

src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp line 458:

> 456: public:
> 457:   ShenandoahControlThread*   control_thread() const    { return _control_thread;    }
> 458: 

Maybe it is easier to `friend` the class that wants it, instead of exposing the control thread for everyone?

src/hotspot/share/gc/shenandoah/shenandoahUtils.hpp line 153:

> 151:     // Shenandoah GC specific safepoints are scheduled by control thread,
> 152:     // so that, querying from control thread can not happen during those
> 153:     // safepoints.

Consider this wording:

    // Shenandoah GC specific safepoints are scheduled by control thread.
    // So if we are enter here from control thread, then we are definitely not
    // at Shenandoah safepoint, but at something else.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1600