RFR (S) 8203479: JFR enabled ARM32 build assertion failure

[Boris Ulasevich]

Hi David,

Many thanks! Here is the werbev with updated copyrights:
http://cr.openjdk.java.net/~bulasevich/8203479/webrev.02

thanks,
Boris

On 18.06.2018 05:39, David Holmes wrote:
> Hi Boris,
> 
> On 15/06/2018 8:44 PM, Boris Ulasevich wrote:
>> Hi,
>>
>> Please review the following patch:
>>    http://cr.openjdk.java.net/~bulasevich/8203479/webrev.01
>>    https://bugs.openjdk.java.net/browse/JDK-8203479
>>
>> Assertion fires in JFR codes on first VM thread setup because VM 
>> globals are not yet initialized (and supports_cx8 property is not 
>> predefined for ARM32 platform). I propose to exploit 
>> early_initialize() method to set up supports_cx8 property on early 
>> stage of VM initialization.
> 
> Your fix looks good. Please update copyright years.
> Just some additional commentary ... from the bug report:
> 
> First _supports_cx8 usage:
>    Threads::create_vm -> JavaThread::JavaThread(bool) --> 
> Thread::Thread() --> JfrThreadLocal::JfrThreadLocal() --> 
> atomic_inc(unsigned long long volatile*)
> 
> I'm not sure when this was introduced but it's risky to perform atomic 
> operations on jlong early during VM initialization. 

For me it comes from revision when JFR was opensourced:

changeset:   50113:caf115bb98ad
user:        egahlin
date:        Tue May 15 20:24:34 2018 +0200
summary:     8199712: Flight Recorder

> Apart from the 
> problem you encountered with ARM32, on some platforms it may require 
> that the stub-generator has been initialized. (Though in that case we 
> may harmlessly fallback to using non-atomic operations - which is okay 
> when creating the initial JavaThread.)

Good point! Yes, we discussed as an optional solution: (1) reworking jfr 
next_thread_id() function to increase counter without atomic_inc 
function call when counter value is zero, and (2) disable assertion when 
!is_init_completed(). We decided that for our case initialization 
sequence reorder works better.

> Thanks,
> David
> 
>> Thanks
>> Boris