RFR: 8217309: ZGC: Fix ZNMethodTable corruption

[Stefan Karlsson]

Hi Per,

On 2019-01-22 10:36, Per Liden wrote:
> Calls to CollectedHeap::register_nmethod()/unregister_nmethod() should 
> be protected by the CodeCache_lock (when called outside of safepoints). 
> However, nmethod::make_unloaded() calls 
> CollectedHeap::unregister_nmethod() without this lock held, which can 
> corrupt the ZNMethodTable. Adding the lock is straight forward. The rest 
> of the patch adjusts ZNMethodTable::unregister_nmethod() to never return 
> false (which should never happen), adds the the proper asserts, etc.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8217309
> Webrev: http://cr.openjdk.java.net/~pliden/8217309/webrev.0

Looks good.

One thought about related code:
    while (_iter_table != NULL) {
      MutexUnlockerEx mu(CodeCache_lock, Mutex::_no_safepoint_check_flag);
      os::naked_short_sleep(1);
    }

We might want to consider changing this into using wait/notify instead 
of unlocker/sleep.

Thanks,
StefanK

> 
> Testing: Tier1-4 on all archs. Tier-1-7 on Linux/x86
> 
> (I'll update the bug with the proper fix-request label, justification, etc)
> 
> /Per