RFR: 8221456: nmethod::make_unloaded() clears _method member too early

[Erik Osterlund]

Hi Per,

Looks good.

Thanks,
/Erik

> On 26 Mar 2019, at 08:24, Per Liden <per.liden at oracle.com> wrote:
> 
> nmethod::make_unloaded() clears the _method member too early, before passing the nmethod to CollectedHeap::unregister_nmethod(). This is not what happens when an nmethod is unregistered via nmethod::make_not_entrant_or_zombie(). We should align this behavior. Clearing the _method member after it has been unregistered is useful, since the GC can then print the method name/signature in logs, etc. Moving the clearing of _method until after CollectedHeap::unregister_nmethod() should be a safe and uncontroversial thing to do.
> 
> Today, ZGC can crash if -Xlog:gc+nmethod=debug is used and an nmethod is unloaded via nmethod::make_unloaded(), because it tries to log the name of the method.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8221456
> Webrev: http://cr.openjdk.java.net/~pliden/8221456/webrev.0
> 
> /Per