RFR(XXS): 8227338: templateInterpreter.cpp: copy_table() needs to be safer

David Holmes david.holmes at oracle.com
Mon Jul 8 00:08:44 UTC 2019

On 7/07/2019 6:48 pm, Erik Osterlund wrote:
> Yeah that switch statement code and yet another plain non-volatile load/store loop looks like complete nonsense unfortunately. It should at least use Atomic::load/store.
> Fortunately, on x86_64, I believe it will in practice yield word atomic copying anyway by chance. But it should be fixed anyway. *sigh*

The requirement is for atomic word accesses, which properly aligned 
addresses will always yield - we rely on that guarantee throughout to 
avoid word-tearing. The issue is that the loop may be converted to a 
'bulk' copying operation that may not provide atomic word accesses. So 
unless the Atomic::load/store will prevent the loop from being converted 
they are not in and off themselves required for correctness (else we 
need to use them nearly everywhere).

> The real danger is SPARC though and its BIS instructions. I don’t have the code in front of me, but I really hope not to see that switch statement and non-volatile loop in that pd_disjoint_words_atomic() function.

sparc uses the same loop.

Let's face it, almost no body expects the compiler to do these kinds of 
transformations. :(

> And I agree that the atomic copying API should be used when we need atomic copying. And if it turns out the implementation of that API is not atomic, it should be fixed in that atomic copying API.

I agree to some extent, but we assume atomic load/stores of words all 
over the place - and rightly so. The issue here is that we need to hide 
the loop inside an API that we can somehow prevent the C++ compiler from 
screwing up. It's hardly intuitive or obvious when this is needed e.g if 
I simply copy three adjacent words without a loop could the compiler 
convert that to a block move that is non-atomic?

> So I think this change looks good. But it looks like we are not done yet. :c

I agree that changing the current code to use the atomic copy API to 
convey intent is fine.


> Thanks,
> /Erik
>> On 7 Jul 2019, at 02:46, Daniel D. Daugherty <daniel.daugherty at oracle.com> wrote:
>> Added Erik O. to the "To:" list...
>>> On 7/6/19 8:05 PM, Daniel D. Daugherty wrote:
>>>> On 7/6/19 6:06 PM, David Holmes wrote:
>>>> Hi Dan,
>>>>> On 6/07/2019 11:53 pm, Daniel D. Daugherty wrote:
>>>>> Greetings,
>>>>> During the code review for the following fix:
>>>>>       JDK-8227117 normal interpreter table is not restored after single stepping with TLH
>>>>>       https://bugs.openjdk.java.net/browse/JDK-8227117
>>>>> Erik O. noticed a potential race with templateInterpreter.cpp: copy_table()
>>>>> depending on C++ compiler optimizations. The following bug is being used
>>>>> to fix this issue:
>>>>>       JDK-8227338 templateInterpreter.cpp: copy_table() needs to be safer
>>>>>       https://bugs.openjdk.java.net/browse/JDK-8227338
>>>>> Here's the webrev URL:
>>>>> http://cr.openjdk.java.net/~dcubed/8227338-webrev/0_for_jdk14/
>>>> So the original code uses a loop to copy, while the new code calls Copy::disjoint_words_atomic, but the implementation of that on x64 is just a loop same as the original AFAICS:
>>> Yup. I figure Erik O. will jump in here with his reasoning... :-)
>> Thinking about it more... I think the answer is that we are switching to
>> calling code that specifies the type of behavior that we need:
>>      Copy::disjoint_words_atomic()
>> is what we need when we're not at a safepoint. If, down the road, we find
>> that the compiler does something with pd_disjoint_words_atomic() that
>> breaks our expectation for pd_disjoint_words_atomic(), then we fix that
>> version of pd_disjoint_words_atomic() and all the callers will be good
>> again...  Or something like that...
>> The version in src/hotspot/os_cpu/solaris_x86/copy_solaris_x86.inline.hpp
>> happens to be exactly our loop with no switch statement... which is
>> particularly funny given Erik's observations about what at least one
>> Solaris X64 compiler did to loops...
>> Still, I'm just guessing here on a Saturday night... hopefully Erik
>> will chime in here...
>> Dan
>>> Dan
>>>> static void pd_disjoint_words_atomic(const HeapWord* from, HeapWord* to, size_t count) {
>>>> #ifdef AMD64
>>>>    switch (count) {
>>>>    case 8:  to[7] = from[7];
>>>>    case 7:  to[6] = from[6];
>>>>    case 6:  to[5] = from[5];
>>>>    case 5:  to[4] = from[4];
>>>>    case 4:  to[3] = from[3];
>>>>    case 3:  to[2] = from[2];
>>>>    case 2:  to[1] = from[1];
>>>>    case 1:  to[0] = from[0];
>>>>    case 0:  break;
>>>>    default:
>>>>      while (count-- > 0) {
>>>>        *to++ = *from++;
>>>>      }
>>>>      break;
>>>>    }
>>>> #else
>>>> David
>>>> -----
>>>>> This fix has been tested via Mach5 Tier[1-3] on Oracle's usual platforms.
>>>>> Mach5 tier[4-6] is running now. It has also been tested with the manual
>>>>> jdb test from JDK-8227117 using 'release' and 'fastdebug' bits.
>>>>> Thanks, in advance, for questions, comments or suggestions.
>>>>> Dan

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