RFR: 8338417: Explicitly pin a virtual thread before acquiring the JFR string pool monitor [v3]

[David Holmes]

On Thu, 15 Aug 2024 11:20:23 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> 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
>
> Markus Grönlund has updated the pull request incrementally with one additional commit since the last revision:
> 
>   update test comment

Functionally seems fine. Could be optimised a bit.

src/jdk.jfr/share/classes/jdk/jfr/internal/StringPool.java line 86:

> 84: 
> 85:     private static void unpinVirtualThread() {
> 86:         if (Thread.currentThread().isVirtual() && ContinuationSupport.isSupported()) {

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.

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20588#pullrequestreview-2240268782
PR Review Comment: https://git.openjdk.org/jdk/pull/20588#discussion_r1718332618