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List:       openjdk-serviceability-dev
Subject:    Re: RFR: 8257831: Suspend with handshakes [v2]
From:       David Holmes <david.holmes () oracle ! com>
Date:       2021-03-31 23:28:09
Message-ID: c28d877c-a027-d471-2f7e-ed181e0ce048 () oracle ! com
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On 1/04/2021 12:35 am, Robbin Ehn wrote:
> On Wed, 31 Mar 2021 10:48:11 GMT, Robbin Ehn <rehn@openjdk.org> wrote:
> 
> > > Thanks @dcubed-ojdk - no it won't. The problem is that the signal can hit very \
> > > late in a thread's termination process, after the JavaThread destructor has \
> > > executed, so no query of JavaThread state is possible. So we needed something \
> > > in the Thread state and the SR_lock was a good enough proxy for that. It may be \
> > > possible to use a different Thread field (perhaps _ParkEvent but encapsulated \
> > > in a Thread::has_terminated() helper method).
> > 
> > SR_handler is used for OS-level suspend/resume (not to conflict with this \
> > change-set). This feature is only used by JFR (AFAIK), and JFR only samples \
> > threads on it's ThreadsList. This means the JavaThread can never be terminated, \
> > hence this code will always pass. 
> > If someone else is using OS-level suspend/resume without a ThreadsList, the bug \
> > is there is no ThreadsList AFAICT. 
> > Since ThreadLocalStorage::thread() is cleared last in ~Thread with \
> > Thread::clear_thread_current(); may be in the ~Thread destructor. So the argument \
> > is that would be safe to read stuff from Thread but not JavaThread? Since the \
> > compiler (and CPU) may reorder and optimize away code, so I argue reading from a \
> > half destroyed object is not a great idea. E.g. Monitor* _SR_lock; is not a \
> > volatile pointer; since reading from this memory is UB after destruction, \
> > compiler is free to remove or not publish the store to NULL. 
> > So I suggest either to remove this check, since the only user is using a \
> > ThreadsList and any other should also be using that too. Or call \
> > Thread::clear_thread_current() before the JavaThread destructor is called, that \
> > way we can be certain that there is no UB.
> 
> I got some offline input from David, there seem to be an issue with signal being \
> delivered after the ThreadsListHandle destructor. If that is the case a ThreadsList \
> doesn't help here. 
> So I suggested we keep this out of this change-set and just take another suitable \
> field to mirror what we have today. 
> `ParkEvent * _ParkEvent;` ?

Yes nicely packaged as:

bool Thread::has_terminated() {
   return _ParkEvent == NULL;
}

Also note:

  // It's possible we can encounter a null _ParkEvent, etc., in 
stillborn threads.
   // We NULL out the fields for good hygiene.
   ParkEvent::Release(_ParkEvent); _ParkEvent   = NULL;

the comment is wrong as it can't be NULL even if stillborn. And now we 
NULL it out for good hygiene and as a late-stage termination indicator.

And yes we can make _ParkEvent volatile to dissuade the compiler from 
eliding the NULLing out.

Thanks,
David

> 
> -------------
> 
> PR: https://git.openjdk.java.net/jdk/pull/3191
> 


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