review (M) for 6965184: possible races in make_not_entrant_or_zombie

[Tom Rodriguez]

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.