JDK-8230459: Test failed to resume JVMCI CompilerThread

[Doerr, Martin]

Hi Kim,

thanks for confirming.

http://cr.openjdk.java.net/~mdoerr/8230459_JVMCI_thread_objects/webrev.04/
already avoids access to freed handles.

I don't really like the complexity of this code.
Replacing oops in handles would have been much more simple.
But I can live with either version.

Best regards,
Martin


> -----Original Message-----
> From: Kim Barrett <kim.barrett at oracle.com>
> Sent: Mittwoch, 6. November 2019 04:09
> To: Doerr, Martin <martin.doerr at sap.com>
> Cc: David Holmes <david.holmes at oracle.com>; dean.long at oracle.com;
> Vladimir Kozlov (vladimir.kozlov at oracle.com)
> <vladimir.kozlov at oracle.com>; hotspot-compiler-dev at openjdk.java.net
> Subject: Re: JDK-8230459: Test failed to resume JVMCI CompilerThread
> 
> > On Nov 5, 2019, at 3:40 AM, Doerr, Martin <martin.doerr at sap.com> wrote:
> 
> Coming back in, because this seems to be going off into the weeds again.
> 
> >> I don't understand what you mean. If a compiler thread holds an oop, any
> >> oop, it must hold it in a Handle to ensure it can't be gc'd.
> >
> > The problem is not related to gc.
> > My change introduces destroy_global for the handles. This means that the
> OopStorage portion which has held the oop can get freed.
> > However, other compiler threads are running concurrently. They may
> execute code which reads the oop from the handle which is freed by this
> thread.
> > Reading stale data is not a problem here, but reading freed memory may
> assert or even crash in general.
> > I can't see how OopStorage supports reading from handles which were
> freed by destroy_global.
> 
> So don't do that!
> 
> OopStorage isn't magic. If you are going to look at an OopStorage
> handle, you have to ensure there won't be concurrent deletion. Use
> locks or some safe memory reclamation protocol. (GlobalCounter might
> be used here, but it depends a lot on what the iterations are doing. A
> reference counting mechanism is another possibility.) This is no
> different from any other resource management.
> 
> > I think it would be safe if the freeing only occurred at safepoints, but I don't
> think this is the case.
> 
> Assuming the iteration didn’t happen at safepoints (which is just a way to
> make the iteration and
> deletion not concurrent).  And I agree that isn’t the case with the current
> code.