RFR: 8274301: Locking with _no_safepoint_check_flag does not check NJT

[David Holmes]

On Thu, 14 Oct 2021 21:04:29 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> There are Mutexes that have a safepoint checking rank, that are locked via:
> MutexLocker ml(safepoint_lock, Mutex::_no_safepoint_check_flag);
> 
> These don't assert for the inconsistency because the thread that calls this is a NonJavaThread.
> 
> But if you lock
> MutexLocker ml(nosafepoint_lock);  // default safepoint check
> 
> These *will* assert for the safepoint checking inconsistency even for NonJavaThreads.
> 
> NonJavaThreads don't participate in the safepoint checking protocol.  And some of these safepoint checking locks are shared between java and nonjava threads. So in the existing code, there's a mix of locking with _no_safepoint_check_flag and not.
> 
> This has a wrinkle because if you have a safepoint checking lock, and call wait on it, we now have to handle the case that the caller is a NonJavaThread and should not safepoint check (just like the Mutex::lock call).
> 
> Maybe this shouldn't be fixed, and we should just document the discrepancy.
> 
> Tested with tier1-3.

Hi Coleen,

If a `MutexLocker`/`MonitorLocker` is only acquired by NJT's then the `_no_safepoint_check` flag is redundant. If it is also used by a JT then of course the flag must remain.

You don't need to make any changes to `Monitor::wait()` because NJTs should only be calling `Monitor::wait_without_safepoint_check()`. The `MonitorLocker::wait` call should call the appropriate `Monitor` function based on the flag and/or the type of thread.

Thanks,
David

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PR: https://git.openjdk.java.net/jdk/pull/5959