RFR (S) 8043070: nmethod::verify_interrupt_point() shouldn't enter safepoint

dean.long at oracle.com dean.long at oracle.com
Fri Nov 17 05:42:38 UTC 2017

On 11/16/17 6:32 PM, Igor Veresov wrote:

>> On Nov 16, 2017, at 3:30 PM, dean.long at oracle.com wrote:
>> Hi Igor.  The changes look good.
> Thanks!
>> Thanks for not changing the values for the other states.  It would have broken AOT :-)
> Yes, we’d like in_use to be 0 so it could be in BSS, right?

And because we compare against 0 right now:

     3440:       48 8b 05 39 bb 21 00    mov 0x21bb39(%rip),%rax        
# 21ef80 <A.meth.state>
     3447:       48 85 c0                test   %rax,%rax
     344a:       0f 85 20 b1 00 00       jne    e570 

>> Are there any situations where make_in_use() might need a compiler/memory barrier?
> I don’t think so, is_in_use() actually returns true for not_installed (now it’s _state <= in_use). So as far as other threads are concerned nothing changed.
> There is an exotic possibility that the initial store of not_installed might be reordered with the store of in_use on non-TSO, but there is a mutex unlock between those, so I’m not really concerned about that.
> Are you thinking about any other scenarios?

I was thinking about make_in_use() happening before the verify() call, 
but that would mean an invalid optimization by a broken C++ compiler, right?
>> I was thinking about an alternative approach, at least for this case.  We could have NoSafepointVerififer always set _thread->_allow_safepoint_count (right now it doesn't for product builds), then verify_interrupt_point() could check that flag before taking the path that can safepoint.  Or how about just using nmethodLocker as suggested in 8028001?  Or does having a new state help solve other problems?
> nmethodLocker approach seemed less tidy because I’d have to lock in nmethod::nmethod() and unlock in ciEnv::register_method().
> We could probably introduce sort of an nmethodHandle that we’d return from new_nmethod() but then we’d have to do reference counting of sorts. Anyway, it felt like an additional state is less trouble.

OK, a new state does look like less trouble than nmethodLocker or 


> igor
>> dl
>> On 11/16/17 11:08 AM, Igor Veresov wrote:
>>> The problems here is that the sweeper can transition a newly created nmethod to not_entrant state before it’s installed. This breaks the logic in nmethod::verify_interrupt_point() called at the end of the installation process. As a result locks might be taken in that can safepoint and safepoints are forbidden during the method install.
>>> The solution is to introduce an new nmethod state: not_installed. The goal is to be able to prevent certain things like sweeping or parts of the verification happening during the nmethod installation.
>>> The change passed the failing test and the internal mach5 pre-intergration testing.
>>> Webrev: http://cr.openjdk.java.net/~iveresov/8043070/webrev.00/
>>> JBS: https://bugs.openjdk.java.net/browse/JDK-8043070
>>> Note, this also fixes: https://bugs.openjdk.java.net/browse/JDK-8028001
>>> Thanks!
>>> igor

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