With the fix for JDK-7145569 the method G1CollectedHeap::mark_strong_code_roots() was introduced. However, this method assumes that the work gang has been set up. If we are running with ParallelGCThreads=0 we don't set up the work gang and thus get a null pointer:

This simple JTreg test illustrates the issue:


/**
 * @test Test8000311
 * @key gc
 * @bug 8000311
 * @summary G1: ParallelGCThreads==0 broken
 * @run main/othervm -XX:+UseG1GC -XX:ParallelGCThreads=0 -XX:+ResizePLAB -XX:+ExplicitGCInvokesConcurrent Test8000311
 * @author filipp.zhinkin@oracle.com
 */

import java.util.*;

public class Test8000311 {
  public static void main(String args[]) {
    for(int i = 0; i<100; i++) {
      byte[] garbage = new byte[1000];
      System.gc();
    }
  }
}

It crashes with a null pointer:

# SIGSEGV (0xb) at pc=0xffffffff7d3fadf4, pid=22622, tid=6

si_addr=0x0000000000000000

Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x7fadf4] void G1CollectedHeap::mark_strong_code_roots(unsigned)+0x44;; __1cPG1CollectedHeapWmark_strong_code_roots6MI_v_+0x44
V [libjvm.so+0x7f5f80] void G1CollectedHeap::g1_process_strong_roots(bool,SharedHeap::ScanningOption,OopClosure*,OopsInHeapRegionClosure*,G1KlassScanClosure*,int)+0x5e0;; __1cPG1CollectedHeapXg1_process_strong_roots6MbnKSharedHeapOScanningOption_pnKOopClosure_pnXOopsInHeapRegionClosure_pnSG1KlassScanClosure_i_v_+0x5e0
V [libjvm.so+0x7fccdc] void G1ParTask::work(unsigned)+0x67c;; __1cJG1ParTaskEwork6MI_v_+0x67c
V [libjvm.so+0x7f7500] void G1CollectedHeap::evacuate_collection_set(EvacuationInfo&)+0x418;; __1cPG1CollectedHeapXevacuate_collection_set6MrnOEvacuationInfo__v_+0x418
V [libjvm.so+0x7f1a84] bool G1CollectedHeap::do_collection_pause_at_safepoint(double)+0x824;; __1cPG1CollectedHeapbGdo_collection_pause_at_safepoint6Md_b_+0x824
V [libjvm.so+0x1241a7c] void VM_G1IncCollectionPause::doit()+0x2e4;; __1cXVM_G1IncCollectionPauseEdoit6M_v_+0x2e4
V [libjvm.so+0x123de38] void VM_Operation::evaluate()+0xf8;; __1cMVM_OperationIevaluate6M_v_+0xf8
V [libjvm.so+0x1239dcc] void VMThread::evaluate_operation(VM_Operation*)+0x254;; __1cIVMThreadSevaluate_operation6MpnMVM_Operation__v_+0x254
V [libjvm.so+0x123a83c] void VMThread::loop()+0x654;; __1cIVMThreadEloop6M_v_+0x654
V [libjvm.so+0x1239834] void VMThread::run()+0xe4;; __1cIVMThreadDrun6M_v_+0xe4
V [libjvm.so+0xef8af0] java_start+0x258;; java_start+0x258