RFR: 8215754: ZGC: nmethod is not unlinked from Method before rendezvous handshake

[Per Liden]

On 01/10/2019 02:59 PM, Per Liden wrote:
> On 1/9/19 4:51 PM, Erik Österlund wrote:
>> Hi,
>>
>> During concurrent class unloading in ZGC, the nmethod unloading goes 
>> through 3 phases:
>>
>> 1) Unlink all references to nmethods
>> 2) Rendezvous handshake, to make sure all JavaThreads are in the new 
>> epoch where unlinking is done
>> 3) Purge nmethods (by making them unloaded).
>>
>> However, we unfortunately unliked the code from the Method to the 
>> nmethod during #3 inside of the nmethod entry barrier instead of #1, 
>> causing JavaThreads to still observe these nmethods during phase #3, 
>> racing with make_unloaded(), and causing crashes in the nmethod entry 
>> barrier.
>>
>> The fix for this is to unlink the nmethod from the method during the 
>> first phase. I additionally eagerly unlink it in the nmethod barrier, 
>> to allow calls into unloading nmethods to not get stuck until the GC 
>> comes around to do it.
>>
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8215754
>>
>> Webrev:
>> http://cr.openjdk.java.net/~eosterlund/8215754/webrev.00/
> 
> Looks good!
> 
> I also ran a slightly easier version of this patch, which had the same 
> effect, through tier{1-7} without seeing any issues.

s/easier/earlier/

> 
> /Per
> 
>>
>> Thanks,
>> /Erik