RFR: 8154715: Missing destructor and/or TLS clearing calls for terminating threads

David Holmes david.holmes at oracle.com
Tue May 3 23:39:07 UTC 2016

This needs attention from GC and runtime folk please.

bug: https://bugs.openjdk.java.net/browse/JDK-8154715
webrev: http://cr.openjdk.java.net/~dholmes/8154715/webrev/

tl;dr: ensure ThreadLocalStorage::set_thread(NULL) is always called 
before a thread terminates.


Most system-related threads do not expect to explicitly terminate, 
except sometimes as part of VM termination. Such threads don't have 
their destructors called, but should.

This omission came to light due to the ThreadLocalStorage changes in 
JDK-8132510. As part of that change we deleted the following from the 
termination path of the VMThread:

  // Thread destructor usually does this.

The clearing of TLS seemed irrelevant to the VMThread as it primarily is 
used to aid in JNI attach/detach. However Brian Gardner reported:


a problem on FreeBSD caused by this change and the interaction with the 
POSIX  pthread TLS destructor use introduced by JDK-8033696. Because the 
VMThread terminated without clearing TLS, when the TLS-destructor was 
called it got into a loop which ran four times (as happens on Linux) and 
then prints a warning to the console (which doesn't happen on Linux).

This indicates we need to restore the:


but on further consideration it seems to me that this is not confined to 
the VMThread, and the most appropriate fix would be to always invoke the 
Thread destructor as a thread terminates.


Further investigation shows that calling the Thread destructor in the 
thread as it terminates is not possible:

- VMThread

This is actually destroyed by the thread that terminates the VM, but 
that can happen after it terminates and so we still hit the TLS problem. 
The VMThread may be able to destroy itself today but in the past this 
was not possible (see existing code comment), and in the future it may 
also not be possible - the problem is that the Thread destructor can 
interact with other VM subsystems that are concurrently being torn down 
by the thread that is terminating the VM. In the past this was the 
CodeHeap. So rather than introduce something that is fragile we stick 
with the current scheme but restore the 
ThreadLocalStorage::set_thread(NULL); - note we can't access "this" at 
that time because it may already have been de-allocated.

- WatcherThread

The WatcherThread is never destroyed today but has the same problem as 
the VMThread. We can call the destructor from the VM termination thread 
(and have implemented that), but not from the WatcherThread itself. So 
again we just have to restore the ThreadLocalStorage::set_thread(NULL); 
to fix the potential TLS problem.

- GC Threads

There are two cases:

a) GC threads that never terminate

For these we don't need to do anything: we can't delete the thread as it 
never terminates and we don't hit the TLS problem because it never 
terminates. So all we will do here is add some logic to check (in 
NON_PRODUCT) that we do in fact never terminate.

b) GC threads that can terminate

Despite the fact the threads can terminate, references to those threads 
are stored elsewhere (WorkGangs and other places) and are not cleared as 
part of the termination process. Those references can be touched after 
the thread has terminated so we can not call the destructor at all. So 
again all we can do (without some major thread management reworking) is 
ensure that ThreadLocalStorage::set_thread(NULL); is called before the 
thread actually terminates

Testing: JPRT
          RBT - runtime nightly tests


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