review (M) for 6965184: possible races in make_not_entrant_or_zombie

[Vladimir Kozlov]

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.
>