Re: RFR: 8287496: Alternative virtual thread implementation that maps to OS thread

[ExE Boss]

On Sun, 29 May 2022 14:46:39 GMT, Alan Bateman <al...@openjdk.org> wrote:

> This patch adds an alternative virtual thread implementation where each 
> virtual thread is backed by an OS thread. It doesn't scale but it can be used 
> by ports that don't have continuations support in the VM. Aside from 
> scalability, the lack of continuations support means:
> 
> 1. JVM TI is not supported when running with --enable-preview (the JVM TI 
> spec allows for this) 
> 2. jshell --enable-preview can't be used (as jshell uses the debugger APIs 
> and so needs JVM TI)
> 
> The VM option "VMContinuations" is added as an experimental option so it can 
> be used by tests. A number of tests are changed to re-run with 
> -XX:-VMContinuations. A new jtreg property is added so that tests that need 
> the underlying VM support to be present can use "@requires vm.continuations" 
> in the test description. A follow-up change would be to add "@requires 
> vm.continuations" to the ~70 serviceability/jvmti/vthread that run with 
> preview features enabled.

Since the package `jdk.internal.access` is exported[^1] to the 
`java.management` module, this can use `MethodHandle`s obtained using the 
trusted lookup.

[^1]: 
https://github.com/openjdk/jdk/blob/73ba7fdce838ba8a2c227a972c176311e6cc0b41/src/java.base/share/classes/module-info.java#L151-L154

src/java.management/share/classes/java/lang/management/ThreadInfo.java line 971:

> 969:             throw new InternalError(e);
> 970:         }
> 971:     }

Suggestion:

    private static boolean isVirtual(Thread thread) {
        try {
            return (boolean) IS_VIRTUAL.invokeExact(thread);
        } catch (Error | RuntimeException e) {
            throw e;
        } catch (Throwable t) {
            throw new InternalError(t);
        }
    }

    private static final MethodHandle IS_VIRTUAL;
    static {
        final JavaLangInvokeAccess JLIA = 
SharedSecrets.getJavaLangInvokeAccess();
        try {
            MethodHandle m = JLIA.findVirtual(Thread.class, "isVirtual", 
MethodType.methodType(boolean.class));
            assert m != null;
            IS_VIRTUAL = m;
        } catch (Exception e) {
            throw new InternalError(e);
        }
    }

src/java.management/share/classes/sun/management/ThreadImpl.java line 661:

> 659: 
> 660:     private static final Method THREAD_IS_VIRTUAL = threadIsVirtual();
> 661:     private static final Field THREADINFO_VIRTUAL = threadInfoVirtual();

Suggestion:

    private static boolean isVirtual(Thread thread) {
        try {
            return (boolean) THREAD_IS_VIRTUAL.invokeExact(thread);
        } catch (Error | RuntimeException e) {
            throw e;
        } catch (Throwable t) {
            throw new InternalError(t);
        }
    }

    /**
     * Returns true if the given ThreadInfo is for a virutal thread.
     */
    private static boolean isVirtual(ThreadInfo threadInfo) {
        try {
            return (boolean) THREADINFO_VIRTUAL.invokeExact(threadInfo);
        } catch (Error | RuntimeException e) {
            throw e;
        } catch (Throwable t) {
            throw new InternalError(t);
        }
    }

    private static final JavaLangInvokeAccess JLIA = 
SharedSecrets.getJavaLangInvokeAccess();

    private static MethodHandle threadIsVirtual() {
        try {
            MethodHandle m = JLIA.findVirtual(Thread.class, "isVirtual", 
MethodType.methodType(boolean.class));
            assert m != null;
            return m;
        } catch (Exception e) {
            throw new InternalError(e);
        }
    }

    @SuppressWarnings("removal")
    private static MethodHandle threadInfoVirtual() {
        PrivilegedExceptionAction<Field> pa = () -> 
ThreadInfo.class.getDeclaredField("virtual");
        try {
            return JLIA.unreflectField(AccessController.doPrivileged(pa), 
false);
        } catch (Exception e) {
            throw new InternalError(e);
        }
    }

    private static final MethodHandle THREAD_IS_VIRTUAL = threadIsVirtual();
    private static final MethodHandle THREADINFO_VIRTUAL = threadInfoVirtual();

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

PR: https://git.openjdk.java.net/jdk/pull/8939