review (M) for 6965184: possible races in make_not_entrant_or_zombie

[Tom Rodriguez]

Thanks!

tom

On Jul 6, 2010, at 2:06 PM, Vladimir Kozlov wrote:

> Then, I think, it is good to push.
> 
> Thanks,
> Vladimir
> 
> Tom Rodriguez wrote:
>> No just those lines testing invocations for 0 changed to 1.  Everything else stayed the same.
>> tom
>> On Jul 6, 2010, at 12:55 PM, Vladimir Kozlov wrote:
>>> Looks good.
>>> 
>>> Did you change anything else I need to review in addition to _invocations at lines 161 and 178 in sweeper.cpp?
>>> 
>>> Thanks,
>>> Vladimir
>>> 
>>> Tom Rodriguez wrote:
>>>> I've correct those uses of _invocations and regenerated the webrev.  I'm rerunning my tests.
>>>> tom
>>>> On Jul 2, 2010, at 5:18 PM, Vladimir Kozlov wrote:
>>>>> Tom,
>>>>> 
>>>>> Nice VTune cleanup :)
>>>>> 
>>>>> Thank you for consolidating sweep logging code.
>>>>> 
>>>>> In nmethod.cpp why you don't use the_method() in your new code (// Remove nmethod from method) ?
>>>>> 
>>>>> In sweeper.cpp why you need second (_invocations > 0) check? Only current thread will modify _invocations now after your changes.
>>>>> 
>>>>> Why you replaced next code:
>>>>> 
>>>>> !   // We want to visit all nmethods after NmethodSweepFraction invocations.
>>>>> !   // If invocation is 1 we do the rest
>>>>> !   int todo = CodeCache::nof_blobs();
>>>>> !   if (_invocations > 1) {
>>>>> !     todo = (CodeCache::nof_blobs() - _seen) / _invocations;
>>>>> !   }
>>>>> ...
>>>>> !     for(int i = 0; i < todo && _current != NULL; i++) {
>>>>> 
>>>>> whith this:
>>>>> 
>>>>> !   int todo = (CodeCache::nof_nmethods() - _seen) / _invocations;
>>>>> ...
>>>>> !     // The last invocation iterates to the end of the code cache
>>>>> !     for (int i = 0; (i < todo || _invocations == 0) && _current != NULL; i++) {
>>>>> 
>>>>> First, _invocations can't be 0 according to the code in possibly_sweep()
>>>>> Second, if it was 0 then you will get division by 0 in todo expression.
>>>>> 
>>>>> Next assert also looks strange:
>>>>> +   assert(_invocations > 0 || _current == NULL, "must have scanned the whole cache");
>>>>> 
>>>>> Vladimir
>>>>> 
>>>>> Tom Rodriguez wrote:
>>>>>> http://cr.openjdk.java.net/~never/6965184
>>>>>> 6965184: possible races in make_not_entrant_or_zombie
>>>>>> Reviewed-by:
>>>>>> While investigating the sweeper concurrency problems I identified a
>>>>>> bunch of minor issues that could lead to races.  NMethod groups a
>>>>>> bunch of flags into a struct using bitfield which was okay in the past
>>>>>> because they were either written by the constructor, at a safepoint or
>>>>>> with a single lock held.  Now some of those fields are written under
>>>>>> one lock and some are written under another so they should be
>>>>>> separated.  I've done away with nmFlags all together and moved a bunch
>>>>>> of common initialization into a new method.  I also deleted a other of
>>>>>> unused or dead things.
>>>>>> In make_not_entrant_or_zombie, all the zombie logic used to be
>>>>>> performed at a safepoint but now it can happen concurrently so the
>>>>>> logic that is executed outside of the Patching_lock has to be more
>>>>>> careful.  All the not_entrant logic has been moved into the region
>>>>>> protected by the Patching_lock.  The link between the nmethod and the
>>>>>> methodOop is also broken ealier though a strong reference to the
>>>>>> methodOop is maintained to make sure it stays alive.
>>>>>> There were problems with the maintenance of _sweep_started and
>>>>>> _invocations that could mistakenly allow another thread into
>>>>>> possibly_sweep but only if there was no work to be done.  Also threads
>>>>>> besides the sweeping thread could clear _sweep_started.
>>>>>> The sweeper divides the cache based on the number of blobs even though
>>>>>> it only works on nmethod, so I added a nof_nmethods count along with a
>>>>>> nof_adapters count just for informational purposes.
>>>>>> As a cleanup I also deleted the dead and unused VTune logic since it
>>>>>> doesn't work and VTune uses JVMTI agents these days.
>>>>>> Tested with NSK jvmti,jdi,jdb,hprof,jit,regression and JDI_REGRESSION
>>>>>> tests.