RFR (XS) 6471769: Error: assert(_cur_stack_depth == count_frames(), "cur_stack_depth out of sync")

serguei.spitsyn at oracle.com serguei.spitsyn at oracle.com
Mon Feb 3 12:13:02 PST 2014


Thank you for reviewing!
Some comments are below.

On 2/3/14 7:51 AM, Daniel D. Daugherty wrote:
> On 1/31/14 7:58 PM, serguei.spitsyn at oracle.com wrote:
>> Please, review the fix for:
>>   https://bugs.openjdk.java.net/browse/JDK-6471769
>> Open webrev:
>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2014/hotspot/6471769-JVMTI-FRAME/ 
> src/share/vm/prims/jvmtiEnv.cpp
>     Thumbs up on the code change.
>     I've never been a fan of the:
>         if (is_thread_fully_suspended(...))
>             do-something-direct
>         else
>             do-something-via-a-safepoint
>     model. We don't have any mechanism in the VM to keep the
>     is_thread_fully_suspended() constraint true over the
>     execution time of the "do-something-direct" operation so
>     we've always been exposed to races.

Thank you for this confirmation.
So that, we are in sync here.

> Also, I think this
>     model is used for several other JVM/TI calls so you might
>     want to visit those also with an eye on safety.

Yes, I'm looking to isolate and double check other places like this in 
the code.
Another piece of work is to isolate other bugs on this topic.

>     For this specific change, I think the comment would be
>     better as:
>     // It is only safe to perform the direct operation on the current
>     // thread. All other usage needs to use a vm-op for safety.

It is taken, thanks!

> In particular, I don't agree with this part of the comment:
>     // Performance-wise the only important case is when current thread is
>     // the target thread.
>     I'm not sure how you can state that performance on the current thread
>     is the only important case, but maybe I don't understand what you're
>     really trying to say here.

I agree, the comment is not clear.
Probably, the comment had to say: "the only case where we can keep 
without loosing safety is the target thread is current ...".
In fact, collecting data at a safepoint is the only way to make it safe.
If the target thread is not current then:
  - a) thread has been suspended: already slow path, so that performance 
is not that critical
  - b) thread has not been suspended: the information we provide is 
         safepoint is the only way to get it safe (is is correct?)

> Now for the more general question with respect to this bug: 6471769
> I've having trouble connecting this change (which I like) to this
> particular bug report.
> The crash in the bug's description looks like this:
> # Internal Error 
> (/net/prt-solamd64-q1-2/PrtBuildDir/workspace/src/share/vm/prims/jvmtiThreadState.cpp, 
> 273), pid=9561, tid=2
> #
> # Java VM: Java HotSpot(TM) 64-Bit Server VM 
> (20060914135846.dcubed.service_hs_b02_merge.2-debug compiled mode)
> #
> # Error: assert(_cur_stack_depth == count_frames(),"cur_stack_depth 
> out of sync")
> V [libjvm.so+0x1217206];; void VMError::report_and_die()+0x606
> V [libjvm.so+0x667841];; void report_assertion_failure(const 
> char*,int,const char*)+0x61
> V [libjvm.so+0xca9557];; int JvmtiThreadState::cur_stack_depth()+0x6e7
> V [libjvm.so+0xc1caaa];; void 
> JvmtiExport::post_method_exit(JavaThread*,methodOop,frame)+0x21ba
> V [libjvm.so+0x8385de];; void 
> InterpreterRuntime::post_method_exit(JavaThread*)+0x21e
> j java.io.ObjectInputStream$BlockDataInputStream.getBlockDataMode()Z+4
> In the above crash, the current thread is trying to post a
> JVMTI_EVENT_METHOD_EXIT and during the event posting code, it
> calls cur_stack_depth() which fails an assert because the
> cached stack depth doesn't match the current queried value:
>    282  int JvmtiThreadState::cur_stack_depth() {
>    283    uint32_t debug_bits = 0;
>    284    guarantee(JavaThread::current() == get_thread() ||
>    285      JvmtiEnv::is_thread_fully_suspended(get_thread(), false, 
> &debug_bits
> ),
>    286      "must be current thread or suspended");
>    287
>    288    if (!is_interp_only_mode() || _cur_stack_depth == 
>  {
>    289      _cur_stack_depth = count_frames();
>    290    } else {
>    291      // heavy weight assert
>    292      assert(_cur_stack_depth == count_frames(),
>    293             "cur_stack_depth out of sync");
>    294    }
>    295    return _cur_stack_depth;
>    296  }
> This is not an unsafe query from one thread to another target thread
> so I don't see how the proposed change will address this failure mode.
> In particular, the assertion is troubling because it tells me that
> the cached stack depth for the current thread is either wrong or has
> been corrupted.

The above are consequences of unsafe operations we are doing.
Current thread should not calculate frame count of target thread (if 
different) other than at a safepoint.
We assume the thread is suspended but, in fact, it was not.
So that we lost safety here and all manifestation we observe are the 

This bug covers the frames miscount issue.
In the fastdebug mode we may get the assert as in this bug report:

There is another potential race that I forgot to check and fix but David 
pointed out.
I will try to cover it in the next version of webrev.

It is still possible there can be other issues here (as you say).
But it is hard to make sure it is the case because it is extremely hard 
to reproduce (you know it!).
I'd suggest to resolve it step-by-step:
   - fix the known safety issues
   - watch the nightly if anything else is left, file and fix new bugs

> To reiterate:
> - I like the code change!
> - I would like to see the comment tweaked a bit
> You should consider going forward with this change using a different
> bug ID, perhaps something like:
>    JDK-NNNNNNN improve safety of JVM/TI GetFrameCount()

There is already bug on this topic (and there are even more dups):

But I still believe, the issue is the same ...


> Dan
>> Summary:
>>   There is a general issue in the suspend equivalent condition 
>> mechanism:
>>   Two subsequent calls to the JvmtiEnv::is_thread_fully_suspended() 
>> may return different results:
>>     - 1-st: true
>>     - 2-nd: false
>>   This more generic suspend equivalent issue is covered by another bug:
>>     https://bugs.openjdk.java.net/browse/JDK-6280037
>>   The bug to fix in this review is a specific manifestation of the 
>> 6280037
>>   in the JVMTI GetFrameCount() that has a big impact on the SQE nightly.
>>   It is on the Test Stabilization radar (as well as the 6280037).
>>   There are many tests intermittently failing because of this.
>>   The webrev for review is a one-liner work around the 6280037 for 
>> the GetFrameCount().
>>   The JVMTI GetFrameCount() spec tells:
>>     "If this function is called for a thread actively executing 
>> bytecodes (for example,
>>      not the current thread and not suspended), the information 
>> returned is transient."
>>   So, it is Ok to call the GetFrameCount() for non-suspended target 
>> threads.
>>   To achieve safety, the frame count for non-suspended threads is 
>> calculated at a safepoint.
>>   It should be Ok and more safe to do the same for suspended threads 
>> as well.
>>   There is no big performance impact because it is already on a slow 
>> path.
>>   It is still important to avoid safepointing when the target thread 
>> is current.
>>   The bug 6280037 should go out of the Test Stabilization radar 
>> (remove the svc-nightly label)
>>   as the most of the impacted tests are covered by the 6471769.
>> Testing:
>>   In progress: nsk.jvmti, nsk.jdi, nsk.jdwp, impacted JTreg tests
>> Thanks,
>> Serguei

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