[9] RFR (XS): 8036588 : VerifyFieldClosure fails instanceKlass:3133

[David Chase]

Thanks much (and Igor also, and John Rose, who I just now realize I left off the list of reviewers), it is now in the hands of the Meddling Medium.

On 2014-07-11, at 1:50 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:

> Looks good.
> 
> Vladimir
> 
> On 7/11/14 9:34 AM, David Chase wrote:
>> How's this look?
>> 
>> http://cr.openjdk.java.net/~drchase/8036588/webrev.01/
>> 
>> Retested with jtreg, also (this time) 900 reps of the bug-tickling test with no error.
>> 
>> David
>> 
>> On 2014-07-09, at 7:02 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:
>> 
>>> Yes, the return_value_is_used() method is incorrect, it does not take into account that the returned value could be used after deoptimization for reallocated objects.
>>> 
>>> return_oop is checked by deoptimization only when EliminateAllocations is true. Only in such case and when objects are reallocated GC can happen during deoptimization.
>>> 
>>> The fix will also preserve a return value when it is really not used:
>>> 
>>> A a = new A();
>>> (void)foo();
>>> a.f = 1;
>>> 
>>> or simple
>>> 
>>> (void)foo();
>>> 
>>> Which is fine since deoptimization code is not critical for performance.
>>> 
>>> return_value_is_used() is used only by this code. I would suggest to rename it to returns_pointer() and copy-paste code from CallNode::returns_pointer() (MachCallNode is not based on CallNode). And use it in the check.
>>> 
>>> Thanks,
>>> Vladimir
>>> 
>>> On 7/8/14 12:13 PM, David Chase wrote:
>>>> 
>>>> bug: https://bugs.openjdk.java.net/browse/JDK-8036588 (closed because only seen in SQE)
>>>> 
>>>> webrev: http://cr.openjdk.java.net/~drchase/8036588/webrev.00/
>>>> 
>>>> cause+fix:
>>>> The root cause is use of the wrong liveness information at deoptimization point.
>>>> The old code uses the optimizer's notion of "live" -- but deoptimization transfers to the
>>>> interpreter and which can (will) manipulate values that are dead to the optimizer.
>>>> The trigger is very tricky -- the following things need to happen:
>>>> 
>>>> 1) an object D that will be dead is allocated
>>>> 2) a method M is invoked that returns an object F, to only be stored in a field f of D
>>>> 3) the optimizer eliminates the allocation of D and the storefield into D.f
>>>> 4) deoptimization hits an execution of M; deoptimization reallocates D for the
>>>>     interpreter; BUT the reallocation triggers a GC, which would forward F if
>>>>     it had been correctly noted as live out of the call to M (but the bug is that it
>>>>     was not).
>>>> 5) the interpreter evaluates D.f = F (this succeeds)
>>>> 6) before the frame with D in it exits, ANOTHER garbage collection occurs (or perhaps
>>>>      GC was running concurrently in some way) and attempts to trace/copy through
>>>>      D and D.f.
>>>> 7) Hilarity ensues.
>>>> 8) For extra giggles, this has only ever been observed with -Xmx=32G (or the corresponding
>>>>      -XX:MaxRAMFraction= option) plus of course -XX:+DeoptimizeALot.  Also setting
>>>>     -XX:DeoptimizeALotInterval=1 increases the failure rate to about 10% of test runs.
>>>>     There's some additional missing context, because following this recipe to write a simpler
>>>>     test for public consumption did not result in a crashing program.
>>>> 
>>>> fix: Use a simpler test for "pointer is live from M" -- if the return type is an object,
>>>>       then it is "live", at least for the interpreter.
>>>> 
>>>> testing:
>>>>   jtreg of runtime, gc, compiler
>>>> 
>>>>   got to the point where I could see fails often enough for the two tests known to trigger this,
>>>>   and after the fix neither test was observed to fail even once, even with hundreds of repetitions.
>>>> 
>> 

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