RFR: 8246442: nmethod::can_convert_to_zombie() asserts when not called by the sweeper

[Erik Österlund]

Hi Vladimir,

Thanks for the review.

/Erik

On 2020-06-03 17:20, Vladimir Kozlov wrote:
> +1
>
> Thanks,
> Vladimir
>
> On 6/3/20 4:40 AM, David Holmes wrote:
>> Hi Erik,
>>
>> LGTM.
>>
>> Your explanation and solution seems entirely reasonable.
>>
>> Thanks,
>> David
>>
>> On 3/06/2020 9:26 pm, Erik Österlund wrote:
>>> Hi,
>>>
>>> nmethod::can_convert_to_zombie() used to be called only by the 
>>> sweeper, after it has checked that
>>> an nmethod is not entrant. Due to the single call site, there was an 
>>> assert checking that the nmethod
>>> should be not entrant (the condition just checked).
>>> However, it is now used in 
>>> nmethod::post_compiled_method_load_event(), where the result of the 
>>> assert is racy.
>>>
>>> When an nmethod is created, the load event will never be able to see 
>>> the nmethod spuriously flipping
>>> to zombie, because there simply are no safepoint polls/transitions 
>>> between the nmethod being created
>>> and the load event firing. However, the jvmti_GenerateEvents can 
>>> observe such spurious flips to zombie.
>>> It is completely harmless, because the whole operation runs under 
>>> the CodeCache_lock, which prevents
>>> further nmethods from being flushed. But they can flip to zombie, 
>>> which is indeed exactly what the
>>> check is design to test. Therefore, this is just a false positive 
>>> assertion failure.
>>>
>>> Proposed fix: make the assert only apply when called from the 
>>> sweeper thread.
>>>
>>> Webrev:
>>> http://cr.openjdk.java.net/~eosterlund/8246442/webrev.00/
>>>
>>> Bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8246442
>>>
>>> Thanks,
>>> /Erik