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

[Thomas Stüfe]

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."

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.

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>
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
>
>