RFR: 8350765: Need to pin when accessing thread container from virtual thread

[Alan Bateman]

On Mon, 12 May 2025 09:01:34 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

>> Data structures that are accessed by both virtual threads and their carriers require the virtual thread to pin the continuation to avoid potential deadlock. A deadlock can arise when a virtual thread is preempted, is selected and scheduled to be the next owner of the lock/resource, but can't execute because all carriers are blocking on the same lock/resource. There are a small number of places that need to pin. One that was missed is the the notification to the thread container when threads are started or terminate. This is not currently an issue at this time but it is a potential hazard for ongoing and future work that will add further scheduling points to the code.  Continuation.pin/unpin have intrinsics since JDK-8338745, and Continuation is initialized early in startup. Finally, the changes have been in the loom repo for several months with no issues.
>> 
>> Testing tier1-5, quick statup/footprint, noreg-hard
>
> src/java.base/share/classes/jdk/internal/vm/ThreadContainer.java line 97:
> 
>> 95:     public final void add(Thread thread) {
>> 96:         // Prevent a virtual thread from being preempted as this could potentially
>> 97:         // deadlock with a carrier is removing a virtual thread from the container
> 
> Is there a typo in this sentence? Should "with a carrier" have been "when a carrier ..." ? Similar comment in the remove(...) method too.

You are right, there is a pre-existing typo in that comment (the sentence was copied to here from RQ).

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PR Review Comment: https://git.openjdk.org/jdk/pull/23810#discussion_r2084260621