RFR: 8347408: Create an internal method handle adapter for system calls with errno [v6]

[Alan Bateman]

On Tue, 14 Jan 2025 07:31:08 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> src/java.base/share/classes/jdk/internal/foreign/CaptureStateUtil.java line 214:
>> 
>>> 212:     // Used reflectively by `getAsIntHandle(MemoryLayout layout)`
>>> 213:     private static int getStateAsInt(VarHandle handle) {
>>> 214:         if (Thread.currentThread().isVirtual() && ContinuationSupport.isSupported()) {
>> 
>> Shouldn't we pin for the entire duration of the native call + errno retrieval? Otherwise, can't we end up in a situation where we have a back to back native call where the second native call overwrites the errno state of the first?
>
> True. We need to rework the solution a bit as we do not want to pin virtual threads during system calls.

A virtual thread can't unmount when there is a native frame on the continuation stack so it's pinned when in the syscall. For the errno capture then I think it will need to explicitly pin before the syscall, then unpin after the capture. The only access to the carrier local will be while pinned.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22391#discussion_r1914451688