RFR: 8205957: setfldw001/TestDescription.java fails with bad field value

[Leonid Mesnik]

On Wed, 14 Aug 2024 19:34:26 GMT, Leonid Mesnik <lmesnik at openjdk.org> wrote:

> The summary of the problem. Test is intermittently failing because can't get expected field watch event.
> The test is failing to get event in the 'setfmodw001b' thread with Xcomp only.
> I verified that frame with the method 'run' of setfmodw001b is compiled when line
> 118: setfmodw001.setWatch(4);
> is executed, however the thread is in interp_only mode. The watch events are supported by interpreter only and just ignored by compiled code.
> 
> The reason of failure is race beteween setting interp_only mode in line
> 
> https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/vmTestbase/nsk/jvmti/SetFieldModificationWatch/setfmodw001.java#L75
> 
>  and calling method call_helper while
>  the method run()
> https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/vmTestbase/nsk/jvmti/SetFieldModificationWatch/setfmodw001.java#L116
> 
>  in newly created thread 'setfmodw001b' is invoked.
> 
> The javaCalls:call are used to invoke methods from hotspot, so it might be rare issues. But still, synchronization might be improved.
> The
> void JavaCalls::call_helper(JavaValue* result, const methodHandle& method, JavaCallArguments* args, TRAPS)
> 
> checks if interp_only mode is set and use 'Method::from_interpreted_entry()' if not. However the interp_only might be set later before compiled method is called (or enter first safe point?). This might happens in safepoint during transition via handshake.
> So the running thread is in interp_only mode however the run() method is compiled and executed already and never going to be deoptimized.
> 
> The additional setWatch calls don't try to deptimize method that are already in interp_only mode.
> 
> BTW, the when JVMCI is enabled and verified adapter exists it is also will be loaded even in interp_only mode set. There is not race here, just ignoring of interp_only mode.
> 
> I run failing test with Xcomp ~1000 times and tiers1-5.

I was able to reproduce failure in "-Xcomp +ZGC'' once in a couple of several hundred runs. Not reproduced anymore with thousands of executions. 
Run tier1-5 to ensure all svc tests passed.

There is no easy way to develop regression test. It should provoke call_helper for compiled method with breakpoint and setting the only single setWatch. Not sure it can find anything in reasonable time.

The call_helper is used only to call methods from hotspot directly, like <clinit> , run() for Thread.start and similar not very common methods. 
I think to review any other possible cases and and thread stack verification that check that interp_only thread don't contain compiled frames on the stack. So we could find similar issues using our testing. Need to find the good place to inject this self-check. 
There are also a couple of places for improvements in this events handling. They are not directly rtelated to this bug. Will file them separately.

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PR Comment: https://git.openjdk.org/jdk/pull/20587#issuecomment-2299317622