12 RFR(M) 8214583: AccessController.getContext may return wrong value after JDK-8212605

[David Holmes]

On 19/12/2018 6:52 am, dean.long at oracle.com wrote:
> David, can I list you as a reviewer?

No, sorry, I only commented on the general issue.

David

> dl
> 
> On 12/16/18 8:47 PM, dean.long at oracle.com wrote:
>> On 12/16/18 7:39 PM, dean.long at oracle.com wrote:
>>> On 12/16/18 7:03 PM, David Holmes wrote:
>>>> On 17/12/2018 12:49 pm, dean.long at oracle.com wrote:
>>>>> On 12/16/18 4:06 PM, David Holmes wrote:
>>>>>> On 15/12/2018 10:59 am, dean.long at oracle.com wrote:
>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8214583
>>>>>>> http://cr.openjdk.java.net/~dlong/8214583/webrev
>>>>>>>
>>>>>>> This change includes two new regression test that demonstrate the 
>>>>>>> problem, and a fix that allows the tests
>>>>>>> to pass.
>>>>>>>
>>>>>>> The problem happens when the JIT compiler's escape analysis 
>>>>>>> eliminates the allocation of the AccessControlContext object 
>>>>>>> passed to doPrivileged.  The compiler thinks this is safe because 
>>>>>>> it does not see that the object "escapes".
>>>>>>
>>>>>> Then surely the compiler's notion of "escapes" needs to be updated!
>>>>>>
>>>>>
>>>>> The compiler can inline the callee method and see that the value 
>>>>> doesn't escape.  This is a valid optimization in cases where the 
>>>>> callee method is known.
>>>>
>>>> But it's not a valid optimization in this case, so my comment stands.
>>>>
>>>> Is this stack walking something this is guaranteed by the spec to be 
>>>> always valid (and hence the JIT is violating the rules), or is the 
>>>> stack walking code making assumptions about whether it will find the 
>>>> context object in the stack?
>>>>
>>>
>>> The stack walking is in the VM and is an internal implementation 
>>> detail, not part of the AccessController API spec.  A different 
>>> thread running normal Java code would never be able to see a 
>>> non-escaping value.  The stack walking code does need to find the 
>>> context object in the stack.  Non-escaping objects won't show up in 
>>> the stack.
>>>
>>>> If we have to hack around this with an annotation I'd rather see a 
>>>> specific annotation that addresses the problematic usecase than a 
>>>> generic "don't inline" one. E.g. @StackVisible or something like that.
>>>>
>>>
>>> That sounds like a good idea for 13, but would require changes to 
>>> both C2 and Graal, and it seems a little risky compared to using 
>>> existing mechanisms.
>>>
>>
>> I forgot to address this in my last reply, but I'm not suggesting a 
>> @DontInline annotation.  That was Claes.  My fixes uses a native method.
>>
>> dl
>>
>>> dl
>>>
>>>> Cheers,
>>>> David
>>>>
>>>>>
>>>>> dl
>>>>>
>>>>>> David
>>>>>> -----
>>>>>>
>>>>>>   However, getContext needs to be able to find
>>>>>>> the object using a stack walk, so we need a way to tell the 
>>>>>>> compiler that it does indeed escape. To do this we pass the value 
>>>>>>> to a native method that does nothing.
>>>>>>>
>>>>>>> Microbenchmark results:
>>>>>>>
>>>>>>> jdk12-b18:
>>>>>>>
>>>>>>> Benchmark                Mode  Cnt    Score   Error Units
>>>>>>> DoPrivileged.test        avgt   25  255.626 ± 6.446 ns/op
>>>>>>> DoPrivileged.testInline  avgt   25  250.968 ± 4.975 ns/op
>>>>>>>
>>>>>>>
>>>>>>> jdk12-b19:
>>>>>>>
>>>>>>> Benchmark                Mode  Cnt  Score    Error Units
>>>>>>> DoPrivileged.test        avgt   25  5.689 ±  0.001 ns/op
>>>>>>> DoPrivileged.testInline  avgt   25  2.765 ±  0.001 ns/op
>>>>>>>
>>>>>>> this fix:
>>>>>>>
>>>>>>> Benchmark                Mode  Cnt  Score    Error Units
>>>>>>> DoPrivileged.test        avgt   25  5.020 ±  0.001 ns/op
>>>>>>> DoPrivileged.testInline  avgt   25  2.774 ±  0.025 ns/op
>>>>>>>
>>>>>>>
>>>>>>> dl
>>>>>
>>>
>>
>