RFR: 8224966: ZGC: Don't skip oop verification in OopMapSet::all_do()

[Stefan Karlsson]

On 2019-05-29 11:27, Per Liden wrote:
> When ZGC is enabled, we skip oop verification in OopMapSet::all_do(). 
> There's no good reason to skip that. The comment suggesting this is 
> skipped because a safepoint can happen between a load and the barrier is 
> incorrect. We can still do the verification as is_in_or_null() doesn't 
> require the address to have a "good" color.

I think the comment is correct. The following situation can cause 
failure in the verification:

register = Object.field
safepoint
load barrier for register

If Object.field points into the old collection set, and we perform a 
mark start or mark end pause, we'll perform a stack walk and hit this 
assert because Heap::is_in_or_null(Object.field) will return false.

I applied the patch and ran stress testing with large collection sets, 
and eventually hit this assert during the mark start phase.

> 
> Also, once JDK-8224675 is integrated, this comment will make even less 
> sense.

Right. After Nils' changes this shouldn't be a problem anymore.

> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8224966
> Webrev: http://cr.openjdk.java.net/~pliden/8224966/webrev.0

Looks good, after Nils' changes have been pushed.

StefanK

> 
> /Per