RFR: 8212663: Remove conservative at_safepoint assert when JFR writes type sets during class unloading

[David Holmes]

Hi Erik,

On 19/10/2018 5:25 PM, Erik Österlund wrote:
> Hi David,
> 
> Isn't the single thread call context the assumed default?

Not sure what you mean.

> In particular, this is called by SystemDictionary::do_unloading() only. 
> And none of that stuff would work if called by multiple threads.
> So while it is indeed possible to distinguish the ZDriver thread as a 
> special type of concurrent GC thread that also handles unloading, and 
> verify there can only be one such thread with that role, for this 
> assert, I'm not sure if it is worth the hassle as all code in the 
> unloading path is single threaded anyway, and I can't see what is 
> special about this code apart from that there was an assert there before 
> and the other single threaded code did not. Do you agree?

The author of the current code considered it desriable to check the code 
was only called at a safepoint. That might mean it only executes in the 
VMThread or it might not - the single-threaded aspect of this code is 
not evident to me just from the code. If checking for a safepoint was 
really a proxy for checking it was executed by the VMThread, and 
concurrent unloading now uses a different thread, then it didn't seem 
unreasonable to ask if the assertion could be modified to cover that 
rather than just removed.

If it's really too much trouble than that's fine, but it was worth 
asking the question.

Cheers,
David


> Thanks,
> /Erik
> 
> On 2018-10-19 03:51, David Holmes wrote:
>> Hi Erik,
>>
>> On 19/10/2018 1:00 AM, Erik Österlund wrote:
>>> Hi,
>>>
>>> JFR writes type sets during class unloading. It currently asserts 
>>> this is done in a safepoint. But in fact, it suffices that it is done 
>>> by a single thread. This assert needs to be relaxed for concurrent 
>>> class unloading.
>>
>> Okay but you removed it completely. Is there not something you can 
>> assert to verify your single thread requirement:
>>
>> Thread::current()->is_VM_thread() || Thread::current()->is_ 
>> classUnloader_thread()
>>
>> ??
>>
>> Cheers,
>> David
>>
>>> Webrev:
>>> http://cr.openjdk.java.net/~eosterlund/8212663/webrev.00/
>>>
>>> Bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8212663
>>>
>>> Thanks,
>>> /Erik
>