RFR (S): 8218483: Crash in "assert(_daemon_threads_count->get_value() > daemon_count) failed: thread count mismatch 5 : 5"

[Thomas Stüfe]

Hi David,


On Tue, Apr 2, 2019 at 10:57 PM David Holmes <david.holmes at oracle.com>
wrote:

> Hi Thomas,
>
> Thanks for taking a look at this.
>
> On 3/04/2019 5:41 am, Thomas Stüfe wrote:
> > Hi David,
> >
> > first thanks for the good analysis!
> >
> > Is this not a problem with the usage of setDaemon():
> >
> >
> https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#setDaemon(boolean)
> >
> > "This method must be invoked before the thread is started."
>
> Not the usage as such, but there is a problem with setDaemon - as per:
>
> https://bugs.openjdk.java.net/browse/JDK-8221657
>
> The test that causes the crash in the VM deliberately tests a case where
> it expects to get the IllegalThreadStateException.
>
> > I think the real solution would be for setDaemon to distinguish between
> > not-yet-started, running and finished. It should not use isAlive(). It
> > should throw an exception if it has been started, regardless of whether
> > it finished already or not.
>
> Yes that fix is needed at the Java level. The use of isAlive() pre-dates
> the existence of Thread.State.
>
> But a change at the Java level may be some time coming given this is a
> day one bug in the spec and implementation of Thread.setDaemon, so I
> wanted to address this quickly in the VM as we are seeing these crashes
> in testing.
>

I think a simple patch could be very simply using

if (threadStatus != 0)

instead of

isAlive()

in Thread.setDaemon?

We do this in other places in Thread.java too.

--

Also I think it makes sense to scan for similar errors in the code base
(isAlive being used as "has-been-started") and fix those too.

For example:

ApplicationShutdownHook.java:

static synchronized void add(Thread hook) {
    if(hooks == null)
        throw new IllegalStateException("Shutdown in progress");

    if (hook.isAlive())
        throw new IllegalArgumentException("Hook already running");

    if (hooks.containsKey(hook))
        throw new IllegalArgumentException("Hook previously registered");

    hooks.put(hook, hook);
}

would register a terminated thread as shutdown hook. I found similar
looking code in ThreadPoolExecutor.

I really think the jdk would be really the right place to fix this.


>
> Thanks,
> David
>
> > Not sure. Its late, I may not be thinking straight.
> >
> > Cheers, Thomas
> >
> >
> >
> > On Tue, Apr 2, 2019 at 12:33 AM David Holmes <david.holmes at oracle.com
> > <mailto:david.holmes at oracle.com>> wrote:
> >
> >     Bug: https://bugs.openjdk.java.net/browse/JDK-8218483
> >     webrev: http://cr.openjdk.java.net/~dholmes/8218483/webrev/
> >
> >     A bug in Thread.setDaemon (JDK-8221657) means that the daemon state
> >     of a
> >     thread can change after the thread is !isAlive() at the Java level.
> If
> >     this happens before the VM call to ThreadService::remove_thread then
> we
> >     have a situation where we incremented the thread counters when the
> >     thread was not a daemon, and we decrement the thread counters when
> the
> >     thread is a daemon - and so the counters are out of sync and the
> >     assertion fires.
> >
> >     The simple fix is to capture the daemon state of the thread while it
> is
> >     still alive and to pass that through to Threads::remove and thus
> >     ThreadService::remove_thread.
> >
> >     Testing:
> >         - manual test with modified VM (to delay Threads::remove call)
> >     as per
> >     the bug report
> >         - mach 5 tiers 1-3
> >
> >     Thanks,
> >     David
> >
>