[16] RFR(S): 8247743: Segmentation fault in debug builds due to stack overflow in find_recur with deep graphs

[Vladimir Kozlov]

Looks good.

Thanks,
Vladimir K

On 7/15/20 12:58 AM, Christian Hagedorn wrote:
> Hi Vladimir
> 
> On 14.07.20 20:46, Vladimir Kozlov wrote:
>> Can you move next up to where other small find*() methods are defined?:
>>
>> +Node* Node::find_ctrl(int idx) {
>> +  return find(idx, true);
>>   }
>>
>> Also add '// not PRODUCT' comment to #endif for #ifndef PRODUCT. It is hard to find where this not product code ends.
>>
>> Looks good otherwise.
> 
> Thanks, I added these changes in a new webrev:
> http://cr.openjdk.java.net/~chagedorn/8247743/webrev.02/
> 
> Best regards,
> Christian
> 
> 
>> Thanks,
>> Vladimir
>>
>> On 7/14/20 2:54 AM, Christian Hagedorn wrote:
>>> Hi Vladimir
>>>
>>> On 13.07.20 19:43, Vladimir Kozlov wrote:
>>>> Node::find_ctrl() is used during debugging when you want to print and look on only control nodes.
>>>> We have several such methods which are only used in debugger.
>>>
>>> I see, I restored this method and changed Node::find() accordingly. I additionally added two find_ctrl() methods to 
>>> make it easier to call it from a debugger (as already present for find_node()).
>>>
>>>> I suggest to store old_arena() in local var and pass into add_to_worklist().
>>>>
>>>> You can make add_to_worklist() static since you pass node as argument.
>>>
>>> Okay. I updated this and the change above in a new webrev:
>>> http://cr.openjdk.java.net/~chagedorn/8247743/webrev.01/
>>>
>>> Best regards,
>>> Christian
>>>
>>>> Thanks,
>>>> Vladimir
>>>>
>>>> On 7/13/20 3:09 AM, Christian Hagedorn wrote:
>>>>> Ping - could anyone review it, please? Thanks!
>>>>>
>>>>> Best regards,
>>>>> Christian
>>>>>
>>>>> On 02.07.20 09:33, Christian Hagedorn wrote:
>>>>>> Hi
>>>>>>
>>>>>> Please review the following patch:
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8247743
>>>>>> http://cr.openjdk.java.net/~chagedorn/8247743/webrev.00/
>>>>>>
>>>>>> The testcase creates a deep graph with a lot of nodes on a chain. When running with the specified test flags, it 
>>>>>> recursively calls Node::find_recur() for each node discovered which eventually results in a segmentation fault due 
>>>>>> to a stack overflow (around 10000 calls due to such a long chain of nodes). The fix just converts the recursive 
>>>>>> algorithm into an iterative one to avoid a segmentation fault. This is similar to JDK-8246203 [1].
>>>>>>
>>>>>> I additionally removed Node::find_ctrl() and its special handling in the algorithm since it is not used.
>>>>>>
>>>>>> There is actually another problem with the recursive version. When running the testcase without 
>>>>>> -XX:CompileOnly=compiler/c2/TestFindNode, it will spin forever inside [2] because there is a debug_orig node cycle 
>>>>>> and the loop does not break based on the debug_orig nodes being visited. This is also fixed in the patch.
>>>>>>
>>>>>> Thank you!
>>>>>>
>>>>>> Best regards,
>>>>>> Christian
>>>>>>
>>>>>>
>>>>>> [1] https://bugs.openjdk.java.net/browse/JDK-8246203
>>>>>> [2] http://hg.openjdk.java.net/jdk/jdk/file/e2622818f0bd/src/hotspot/share/opto/node.cpp#l1589