Request for reviews (XS): 7106944: assert(_pc == *pc_addr) failed may be too strong

[Tom Rodriguez]

On Nov 2, 2011, at 10:22 AM, Vladimir Kozlov wrote:

> Yes, it looks good. But, Christian, can you show (call stack?) how we endup deoptimizing twice? How we enter deoptimization code twice?

I think it's biased locking revocation.  Here's the assertion point:

V  [libjvm.so+0x2701328]  void VMError::report(outputStream*)+0x8c8
V  [libjvm.so+0x2702469]  void VMError::report_and_die()+0x4fd
V  [libjvm.so+0xe869bf]  void report_vm_error(const char*,int,const char*,const char*)+0x55f
V  [libjvm.so+0xff8666]  void frame::patch_pc(Thread*,unsigned char*)+0xe6
V  [libjvm.so+0xfe6e8d]  void frame::deoptimize(JavaThread*)+0x361
V  [libjvm.so+0xed9622]  void Deoptimization::deoptimize_frame(JavaThread*,long*)+0x55e
V  [libjvm.so+0xa34674]  void deopt_caller()+0x154
V  [libjvm.so+0xa35974]  void Runtime1::new_type_array(JavaThread*,klassOopDesc*,int)+0x3c4

deopt_caller has a check for the caller already being deopted:

static bool caller_is_deopted() {
  JavaThread* thread = JavaThread::current();
  RegisterMap reg_map(thread, false);
  frame runtime_frame = thread->last_frame();
  frame caller_frame = runtime_frame.sender(&reg_map);
  assert(caller_frame.is_compiled_frame(), "must be compiled");
  return caller_frame.is_deoptimized_frame();
}

// Stress deoptimization                                                                                                                                
static void deopt_caller() {
  if ( !caller_is_deopted()) {
    JavaThread* thread = JavaThread::current();
    RegisterMap reg_map(thread, false);
    frame runtime_frame = thread->last_frame();
    frame caller_frame = runtime_frame.sender(&reg_map);
    Deoptimization::deoptimize_frame(thread, caller_frame.id());
    assert(caller_is_deopted(), "Must be deoptimized");
  }
}

but down in deoptimize we may revoke biases at a safepoint:

  if (UseBiasedLocking) {
    revoke_biases_of_monitors(thread, fr, map);
  }
  deoptimize_single_frame(thread, fr);

This allows a deopt to occur during the safepoint, after the initial guard.

tom

> 
> Thanks,
> Vladimir
> 
> On 11/2/11 10:01 AM, Tom Rodriguez wrote:
>> Looks good.
>> 
>> tom
>> 
>> On Nov 2, 2011, at 8:53 AM, Christian Thalinger wrote:
>> 
>>> http://cr.openjdk.java.net/~twisti/7106944/
>>> 
>>> 7106944: assert(_pc == *pc_addr) failed may be too strong
>>> Reviewed-by:
>>> 
>>> Sometimes we are already deoptimizing a frame when we request another
>>> deoptimization of the same frame.  This makes the assert to fail.
>>> 
>>> The fix is to add a check to the assert that the return address is the
>>> same that we are going to patch.
>>> 
>>> src/cpu/x86/vm/frame_x86.cpp
>>> 
>>