Deadlock detection mechanism incorrectly assumes that thread cannot block itself

[David Holmes]

On 28/11/2012 2:10 AM, Dmytro Sheyko wrote:
> Hi,
>
> One more patch regarding deadlock detection.
> https://bugs.openjdk.java.net/show_bug.cgi?id=100059
>
> Deadlock detection mechanism assumes that thread cannot block itself. In
> general this is not right, especially in case of non-reentrant mutexes.

Arguably that is not a deadlock per se. I'm unclear in your patch as to 
what conditions actually constitute a self-deadlock and how it is 
detected. For example:

Locksupport.park(); // if no one knows to unpark me I never return

Object o = new Object();
synchronized(o) { o.wait(); } // can't get notified

CountdownLatch l = new CountdownLatch();
l.await(); // no one can count down

Should these be reported? I don't see how they could without much more 
elaborate analysis of the objects involved. So what constituates a 
detectable self-deadlock?

Thanks,
David


> One of such examples we can find here (FIFOMutex sample):
> http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/LockSupport.html
>
> To fix this, we just need to drop unnecessary code.
>
> Attached updated patch and testcase
>
> Thanks,
> Dmytro