8338417: Explicitly pin a virtual thread before acquiring the JFR string pool monitor

[mgronlun]

Greetings,

Explicitly pin a virtual thread before acquiring the JFR string pool monitor because migrating a carrier thread local event writer object onto another carrier thread is impossible.

During event commit, the thread is in a critical section because it has loaded a carrier thread local event writer object. For virtual threads, a contended monitor, such as a synchronized block, is a point where a thread could become unmounted.

A monitor guards the JFR string pool, but remounting a virtual thread onto another carrier is impossible because of the event writer.

Therefore, it's imperative to use explicit pin constructs to prevent unmounting at this location.

Testing: jdk_jfr

Thanks
Markus

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Issue

• JDK-8338417: Explicitly pin a virtual thread before acquiring the JFR string pool monitor (Bug - P3)

Reviewers

• David Holmes (@dholmes-ora - Reviewer)
• Alan Bateman (@AlanBateman - Reviewer) 🔄 Re-review required (review applies to b047b95c)

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8338417: Explicitly pin a virtual thread before acquiring the JFR string pool monitor

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• 74fdd68: 8333791: Fix memory barriers for @stable fields
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• 01: Full - Incremental (b047b95c)
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[dholmes-ora]

Functionally seems fine. Could be optimised a bit.

[dholmes-ora]

If you are at all concerned about overhead here then pin could return a boolean to indicate if the pin happened and oyu could then unpin just by checking that boolean and avoid doing the isVirtual and isSupported checks again.

[egahlin]

Would it be possible to create a boolean in the EventWriter that indicates if it is associated with a carrier thread or a normal thread (which can never be virtual) and then have two methods.

long l = this.carrierThread ? StringPool.addPinnedString(s) : StringPool.addString(s);

[AlanBateman]

Thread.currentThread() has an intrinsic, and isVirtual is just a type check. ContinuationSupport.isSupported reads a static final so will disappear once compiled. The pattern we are using in other areas is for the pin to return a boolean (like David suggested).