[foreign-memaccess+abi] RFR: 8293367: Enable native access for modules not in the boot layer [v2]

[Jorn Vernee]

On Wed, 21 Sep 2022 13:38:33 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Or, maybe it's possible to do this in a lock-free manner altogether? i.e. `implAddEnableNativeAccess` and  would use a volatile write, and `isNativeAccessEnabled` and `ensureNativeAccess` would use volatile reads instead of synchronized blocks. (assuming we can use `Unsafe` at this point).
>
> That's a thought that occurred to me as well. E.g. declare the variable w/o `volatile`, but then use either Unsafe, or VarHandle to access the variable in `volatile` mode when required. That said, note that this will not get rid of all the synchronization needs: we would still need to synchronize on a module in the native access check to make sure that we issue at most one warning. But perhaps we can have a private lock just for that.

I think it would be possible to emit only one warning by using a compare and exchange.


boolean shouldEnable = !ModuleBootstrap.hasEnableNativeAccessFlag();
isNativeAccessEnabled = Unsafe.compareAndExchangeBoolean(target, OFFSET, false, shouldEnable);
// check again with the safely read flag
if (isNativeAccessEnabled) {
    // another thread beat us to it - nothing to do
    return;
} else if (!shouldEnable) {
    throw new IllegalCallerException("Illegal native access from: " + this);
} else {
    // If we get here, compare and exchange above has set enableNativeAccess to true.
    // emit watning
}

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

PR: https://git.openjdk.org/panama-foreign/pull/729