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

[David Chase]

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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