RFR: 8284161: Implementation of Virtual Threads (Preview) [v3]

[Jaikiran Pai]

On Fri, 15 Apr 2022 11:45:01 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> This is the implementation of JEP 425: Virtual Threads (Preview); TBD which JDK version to target.
>> 
>> We will refresh this PR periodically to pick up changes and fixes from the loom repo.
>> 
>> Most of the new mechanisms in the HotSpot VM are disabled by default and require running with `--enable-preview` to enable.
>> 
>> The patch has support for x64 and aarch64 on the usual operating systems (Linux, macOS, and Windows). There are stubs (calling Unimplemented) for zero and some of the other ports. Additional ports can be contributed via PRs against the fibers branch in the loom repo.
>> 
>> There are changes in many areas. To reduce notifications/mails, the labels have been trimmed down for now to hotspot, serviceability and core-libs. We'll add the complete set of labels when the PR is further along.
>> 
>> The changes include a refresh of java.util.concurrent and ForkJoinPool from Doug Lea's CVS. These changes will probably be proposed and integrated in advance of this PR.
>> 
>> The changes include some non-exposed and low-level infrastructure to support the (in draft) JEPs for Structured Concurrency and Scope Locals. This is to make life a bit easier and avoid having to separate VM changes and juggle branches at this time.
>
> Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Refresh
>  - Refresh
>  - Merge with jdk-19+18
>  - Refresh
>  - Initial push

src/java.base/share/classes/java/lang/Thread.java line 602:

> 600:         } else {
> 601:             // getContextClassLoader not trusted
> 602:             ClassLoader cl = parent.contextClassLoader;

I might be misreading the comment on that line, but reading this if/else block a few times, I'm unsure what the expectations here are.

It's my understanding that a call to `getContextClassLoader()` can't be trusted if that method has been overridden by the passed `Thread` type. In such cases, we don't call that method and instead just use the field value of `contextClassLoader` (which is a private field on `Thread`). Is that understanding correct? If yes, then the condition in the `if` block a few lines above, looks odd. It seems to be calling the `getContextClassLoader()` if  that method is overridden by the passed `Thread` type, i.e. the untrusted case. Should it instead be:

if (sm == null || !isCCLOverridden(parent.getClass())) {
    return parent.getContextClassLoader();
}

(notice the negation)

-------------

PR: https://git.openjdk.java.net/jdk/pull/8166