review (M) for 6965184: possible races in make_not_entrant_or_zombie

[Tom Rodriguez]

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.