Shenandoah hangs on specjvm2008 due to reentrant handshake

[Zhengyu Gu]

> 
> Is there a chance that threads will load stale oop in from-space  and
> hold it after fix-ref is down and then store the stale oop into heap before
> entry_update_thread_roots?
> 

No, LRB guarantees no from-space oops can be written to heap when it is 
armed.

> Is there a plan to re-use the barrier data to implement LRB to completely
> avoid "non-atomic" of loads and LRB like ZGC?
>

ZGC uses late barrier expansion to solve the problem, where Shenandoah 
has trouble to do. Roman (cc'd) can explain better.

-Zhengyu

> Thanks,
> Liang
> 
> ------------------------------------------------------------------
> From:Zhengyu Gu <zgu at redhat.com>
> Send Time:2021 May 12 (Wed.) 20:25
> To:"MAO, Liang" <maoliang.ml at alibaba-inc.com>; shenandoah-dev 
> <shenandoah-dev at openjdk.java.net>
> Subject:Re: Shenandoah hangs on specjvm2008 due to reentrant handshake
> 
> Hi Liang,
> 
> 
> On 5/12/21 6:53 AM, Liang Mao wrote:
>  > Hi Shenandoah team,
>  >
>  > We found this issue while running Shenandoah with jdk/master:
>  > https://bugs.openjdk.java.net/browse/JDK-8266963 
> <https://bugs.openjdk.java.net/browse/JDK-8266963> in which we had a simple fix
>  > so far.
> 
> Thanks for reporting. Could you please post your fix?
> 
>  >
>  > The hanging comes from 
> https://bugs.openjdk.java.net/browse/JDK-8262443 
> <https://bugs.openjdk.java.net/browse/JDK-8262443> which
>  > introduced the possibility of reentrant handshake and looks like only shenandoah
>  > would hit the scenario. I have a question specific to shenandoah that why do we
>  >   need the phase of "entry_update_thread_roots"?
>  > I think we already concurrently relocate the oops in thread roots by
>  > "entry_thread_roots".
> 
> entry_thread_roots fixes on-stack frames, but can not prevent thread
> from loading from-space oops during continuing execution. Ideally,
> Shenandoah LRB should prevent this from happening, if LRB is "atomic" (
> no safepoints between load and LRB), unfortunately, current LRB is not.
> 
>  > And "entry_update_thread_roots" doesn't use a general concurrent stack processing
>  > via stack watermark but just a handshake.
> 
> At point we execute entry_update_thread_roots, cset has been completely
> evacuated, therefore, no more from-space oops in heap. There are very
> rare cases that threads can still hold stalled oops, can be flushed out
> very quick, especially, if we can handshake individual Java thread and
> not cause global safepoint.
> 
> -Zhengyu
> 
>  >
>  > Thanks,
>  > Liang
>  >
>