[concurrency-interest] a volatile bug?

Aleksey Shipilev aleksey.shipilev at gmail.com
Wed May 16 15:23:14 EDT 2012


All right, here's what is on the table.

This bug is reproduced for me on Linux i686 with:
java version "1.7.0_04"
Java(TM) SE Runtime Environment (build 1.7.0_04-b20)
Java HotSpot(TM) Client VM (build 23.0-b21, mixed mode)

It reproduces immediately only with -client.
Both -server and -Xint do NOT reproduce the bug.
The code is there in original SO post
http://stackoverflow.com/questions/10620680/why-volatile-in-java-5-doesnt-synchronize-cached-copies-of-variables-with-main

C1 seems to miscompile run(), and indeed does CSE for local:

  # {method} 'run' '()V' in 'Test$1'
[Verified Entry Point]
  0xb4a91e80: mov    %eax,-0x4000(%esp)
  0xb4a91e87: push   %ebp
  0xb4a91e88: sub    $0x18,%esp         ;*invokestatic access$000
                                        ; - Test$1::run at 0 (line 11)
  0xb4a91e8b: mov    $0xa09c4270,%edx   ;   {oop(a 'java/lang/Class' = 'Test')}
>>>>>>  0xb4a91e90: mov    0x74(%edx),%edx    ;*getstatic b <<<<<---- loads $b to %edx
                                        ; - Test::access$000 at 0 (line 1)
                                        ; - Test$1::run at 0 (line 11)
  0xb4a91e93: jmp    0xb4a91e9e         ; OopMap{off=40}
                                        ;*goto
                                        ; - Test$1::run at 10 (line 13)
  0xb4a91e98: test   %eax,0xb77a9100    ;*goto
                                        ; - Test$1::run at 10 (line 13)
                                        ;   {poll}
  0xb4a91e9e: mov    $0xa09c4270,%ecx   ;   {oop(a 'java/lang/Class' = 'Test')}
>>>>>  0xb4a91ea3: mov    0x70(%ecx),%ecx    ;*getstatic a  <<<<< volatile read for $a
                                        ; - Test::access$100 at 0 (line 1)
                                        ; - Test$1::run at 4 (line 13)
  0xb4a91ea6: cmp    $0x0,%ecx     // <---- $a is at %ecx
  0xb4a91ea9: je     0xb4a91e98         ;*ifne
                                        ; - Test$1::run at 7 (line 13)
 >>>> 0xb4a91eab: cmp    $0x0,%edx     // <<<<<<---- $b is cached in %edx here
  0xb4a91eae: jne    0xb4a91ed8         ;*ifne
                                        ; - Test$1::run at 16 (line 17)
  0xb4a91eb4: nopl   0x0(%eax)
  0xb4a91eb8: jmp    0xb4a91f0e         ;   {no_reloc}
  0xb4a91ebd: xchg   %ax,%ax
  0xb4a91ec0: jmp    0xb4a91f28         ; implicit exception:
dispatches to 0xb4a91f18
  0xb4a91ec5: nop                       ;*getstatic out
                                        ; - Test$1::run at 19 (line 18)
  0xb4a91ec6: cmp    (%ecx),%eax        ; implicit exception:
dispatches to 0xb4a91f32
  0xb4a91ec8: mov    $0xa09c6488,%edx   ;*invokevirtual println
                                        ; - Test$1::run at 24 (line 18)
                                        ;   {oop("error")}


Thanks,
Aleksey.

On Wed, May 16, 2012 at 11:12 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> It can be a compiler (mis)optimization that causes this, and not x86 memory
> ordering.
>
> Someone posted the assembly output in the comments on SO and it does seem
> like there's a place that loads 'b' from the stack rather than memory.
> Hans' theory of CSE sounds plausible - can someone repro this without that
> "int tt = b;" line?
>
> Adding hotspot compiler guys in case they want to chime in.
>
> Sent from my phone
>
> On May 16, 2012 3:07 PM, "Aleksey Shipilev" <aleksey.shipilev at gmail.com>
> wrote:
>>
>> On Wed, May 16, 2012 at 10:40 PM, Boehm, Hans <hans.boehm at hp.com> wrote:
>> > A JDK bug AND a serious test suite omission?
>>
>> Stress tests would probably JIT-compile the code in question. See below.
>>
>> > But is the problem real?  Can it be reproduced on a mainstream JVM?
>>
>> Same question.
>>
>> > Note that the example in the original posting also read b before the
>> > loop,
>> > so naïve common subexpression elimination would cause the bug.
>> >  Hopefully
>> > nobody does CSE in cases like this.
>>
>> FWIW, the test case in SO would probably not hit any compilation
>> threshold in HotSpot, so it could be executed in interpreter. Then,
>> assuming the interpreter does not reorder Java code, and assuming
>> original SO poster runs Windows, and hence x86, and hence has TSO,
>> this bug seems very unlikely. I would be surprised if it actually
>> *can* be reproduced. That makes the whole story rather interesting.
>>
>> -Aleksey.
>>
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