Hi Carsten,
On 11/02/2015 8:18 AM, Carsten Varming wrote:
Dear David,
Any news regarding this fix? Anything I can do to help?
The bug has been assigned to Dan Daugherty so it depends on his
workload. Verifying the safety of the change is a time consuming task.
Thanks,
David
Best wishes,
Carsten
On Wed, Feb 4, 2015 at 12:54 PM, Carsten Varming <varm...@gmail.com
<mailto:varm...@gmail.com>> wrote:
Dear David,
I took a look at all occurrences of PeriodicTask_lock. We are in luck:
PeriodicTask::enroll and PeriodicTask::disenroll check for
safepoints when acquiring PeriodicTask_lock.
PeriodicTask::time_to_wait and PeriodicTask::real_time_tick are
called only by the watcher thread.
WatcherThread::unpark is used only in contexts where
PeriodicTask_lock is owned by the caller.
WatcherThread::sleep is called only by the watcher thread.
I took another look at WatcherThread::sleep and realized that there
is a path from acquiring PeriodicTask_lock to waiting on the lock
where _should_terminate is not checked. I added that to the minimal
fix attached.
Looking at these methods made me want to clean up a little more. See
better.patch attached.
I feel pretty good about the patch with the limited usage of
no_safepoint_check_flag with PeriodicTask_lock.
Carsten
On Tue, Feb 3, 2015 at 11:28 PM, David Holmes
<david.hol...@oracle.com <mailto:david.hol...@oracle.com>> wrote:
On 4/02/2015 1:28 PM, Carsten Varming wrote:
Dear David Holmes,
Please see inline response,
On Tue, Feb 3, 2015 at 9:38 PM, David Holmes
<david.hol...@oracle.com <mailto:david.hol...@oracle.com>
<mailto:david.holmes@oracle.__com
<mailto:david.hol...@oracle.com>>> wrote:
On 4/02/2015 5:00 AM, Carsten Varming wrote:
Greetings all,
I was recently introduced to the deadlock described in
JDK-8047720 and
fixed in JDK9. The fix seems a little messy to me,
and it looks
like it
left some very short races in the code. So I
decided to clean up the
code. See attached diff.
The change takes a step back and acquires
PeriodicTask_lock in
WatcherThread::stop (like before the fix in JDK9),
but this time
safepoints are allowed to proceed when acquiring
PeriodicTask_lock,
preventing the deadlock.
It isn't obvious that blocking for a safepoint whilst
in this method
will always be a safe thing to do. That would need to
be examined in
detail - potential interactions can be very subtle.
Absolutely true. For reference, the pattern is repeated with the
Terminator_lock a few lines below. The pattern is also used in
Threads::destroy_vm before and after calling before_exit,
and the java
shutdown hooks are called right before the call to
before_exit. So there
is a lot of evidence that safepoints are allowed to happen
in this context.
The thread calling before_exit is a JavaThread so of course it
participates in safepoints. The issue is whether the interaction
between that thread and the WatcherThread, via the
PeriodicTask_lock, can allow for the JavaThread blocking at a
safepoint whilst owning that lock. If another JavaThread can try
to lock it without a safepoint check then we can get a deadlock.
Cheers,
David
Thank you for taking your time to look at this,
Carsten
David
Let me know what you think,
Carsten
