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

[Per Liden]

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.

/Per

> 
> Thanks,
> /Erik