[16] 8247736: Shenandoah: assert(_nm->is_alive()) failed: only alive nmethods here

[Roman Kennke]

The patch looks ok to me.

Thank you,
Roman

On Mon, 2020-06-22 at 10:12 -0400, Zhengyu Gu wrote:
> The assertion is unreliable, as a nmethod can become a zombie before
> it 
> is unregistered, and nmethod's state change can race against
> concurrent 
> nmethod iteration, since they are under two different locks.
> 
> We did not see this assertion before JDK-8245961, because we used 
> CodeCache::blobs_do() to scan code cache and did not have the
> assertion 
> on its code path.
> 
> Ideally, I would prefer to keep nmethod list hygienic: unregister
> the 
> nmethod before making state transition. However, offline discussion
> with 
> Erik, he convinced me that could have unexpected consequences and
> risky. 
> Mark through and evacuate/disarm zombie nmethods, while undesirable,
> but 
> harmless.
> 
> So, let's just filter out dead nmethod (still racy) and remove the 
> assertion.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8247736
> Webrev: 
> http://cr.openjdk.java.net/~zgu/JDK-8247736/webrev.00/index.html
> 
> Test:
>    hotspot_gc_shenandoah
> 
> Thanks,
> 
> -Zhengyu
>