On 07/05/2013 01:22 AM, David Holmes wrote:
Hi Peter,
On 2/07/2013 5:19 PM, Peter Levart wrote:
Looking at original code once again, I think this was actually a bug.
The WeakReference instance constructed in (old) line 82, can be GCed
right away, since nobody is using the local variable after assignment.
Of course. Doh! I was to busy thinking about the lifetime of the
referent object to consider the reference itself.
If WeakReference is GCed it can not be enqueued. The promotion of local
variable into a field is one way to fix this. The other would be to use
the local variable somewhere down the code path, like for example in a
final throw statement:
110 throw new IllegalStateException("Reference Handler thread
stuck. weakRef.get(): " + weakRef.get());
This would also reveal some more info about the WeakReference when
there's no sure answer after 10 seconds and could be added to the test
anyway.
Okay I've modified the test as suggested. Updated webrev at same
location:
http://cr.openjdk.java.net/~dholmes/8016341/webrev/
In testing it though I simply exposed the remaining flaws in the
ReferenceHandler code. We can still kill the ReferenceHandler thread
with an OOME when it tries to load the Cleaner class (running with a
5M heap triggers this nicely if you use G1):
// Fast path for cleaners
if (r instanceof Cleaner) {
((Cleaner)r).clean();
continue;
}
and if that passes the clean() might throw OOME (it internally tries
to do a System.exit if an exception occurs but will likely encounter
another OOME trying to create the PrivilegedAction).
Even the:
ReferenceQueue q = r.queue;
if (q != ReferenceQueue.NULL) q.enqueue(r);
might throw OOME because enqueue() might have to load the
FinalReference class.
So really catching the OOME around the wait() only patches a small
hole. We can't simply put a try/catch in the for(;;) loop because that
doesn't address the problem that if the class loading throws OOME then
subsequent attempts to load that class will also fail. We would have
to preload all possible classes. Even then we might just send the
ReferenceHandler thread into a busy loop of throwing OOME catching it
and retrying!
So I can fix the test to deal with the Xcomp issue but we may still
see intermittent failures, and the ReferenceHandler thread may still
die from OOME.
Hi David,
I think the test is fine now.
Regarding the resilience of ReferenceHandler thread, I think we should
try to preload FinalReference, Cleaner and InterruptedException in the
initialization phase of ReferenceHandler thread. There should be no
danger of OOME in related cases then. As far as Cleaner's exception
handler is concerned, I think it is not universally wise to just exit
the VM when any exception is thrown by the Cleaner's thunk.run() method.
What if that exception is OOME? That does not mean there's something
wrong with Cleaner.thunk's code. Not only will this kill
ReferenceHandler thread, but entire VM.
If the purpose of exiting VM was attracting attention to the possible
bug in Cleaner.thunk's code, then this absolutely works, but wouldn't
simple message to System.err be enough? Like for example:
public void clean() {
if (!remove(this))
return;
try {
thunk.run();
} catch (final Throwable x) {
try {
new Error("Cleaner caught exception", x)
.printStackTrace();
} catch (OutOfMemoryError oome) {
// can't do much
}
}
}
Regards, Peter
David
-----
Regards, Peter
On 07/02/2013 06:38 AM, David Holmes wrote:
This recently added test was found to fail under some conditions -
namely client compiler with -Xcomp. It seems that the use of all local
variables enabled the compiler to optimize things in a way that
stopped the weakref from being enqueued as expected. Simple fix was to
make the weakref a field.
http://cr.openjdk.java.net/~dholmes/8016341/webrev/
Thanks,
David
