Fwd: Patch: Clean up fix for JDK-8047720

[Carsten Varming]

Dear David Holmes,

Please see inline response,

On Tue, Feb 3, 2015 at 9:38 PM, David Holmes <david.holmes at 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.

Thank you for taking your time to look at this,
Carsten


>
> David
>
>
>  Let me know what you think,
>> Carsten
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/serviceability-dev/attachments/20150203/c82d4355/attachment.html>