[14] RFR(s) : 8234173: assert(loader != __null && oopDesc::is_oop(loader)) failed: loader must be oop

[Kim Barrett]

> On Jan 16, 2020, at 3:25 AM, Markus Gronlund <markus.gronlund at oracle.com> wrote:
> 
> Hi Sangheon,
> 
> What (and if so how) is concurrently reading this? Since the operation of both clearing and resetting the mark word happens in a safepoint (only the VMThread running)?

G1, ZGC, and (we think) Shenandoah all have GC threads that do marking (using side tables,
not the mark words) and maybe other things right on through safepoints.  See, for example,
Thomas’s comment from 1/10 in the bug, about the G1ConcurrentMarkThread.