review (M) for 6965184: possible races in make_not_entrant_or_zombie

[Vladimir Kozlov]

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.