RFR(XXS): 8227117: normal interpreter table is not restored after single stepping with TLH

Erik Österlund erik.osterlund at oracle.com
Thu Jul 4 08:08:58 UTC 2019

Hi David,

When you run without TLH, this copying mechanism is used to synchronize 
the safepoint while JavaThreads are running. The interpreter doesn't 
emit any polls then. Instead it clobbers the dispatch table. JavaThreads 
will be reading from the dispatch table while it is being 
(non-atomically) modified. That could crash. For example with the 
Solaris + studio + SPARC - TLH configuration, the compiler will almost 
certainly emit a memcpy (this transformation has been observed in 
practice), the memcpy will use BIS instructions (observed in practice) 
for performance, with out-of-thin-air values (observed in practice), and 
the JavaThreads will occasionally crash during safepoint synchronization 
due to said out-of-thin-air values.

So I guess the problem might be larger back when TLH was not default. 
But this seems conceptually wrong.


On 2019-07-04 09:17, David Holmes wrote:
> Hi Erik,
> On 4/07/2019 5:10 pm, Erik Österlund wrote:
>> Hi Dan,
>> Thanks for picking this up. The change looks good.
>> However, when reviewing this, I looked at the code for actually 
>> restoring the table (ignore/notice safepoints). It copies the 
>> dispatch table for the interpreter. There is a comment stating it is 
>> important the copying is atomic for MT-safety, and I can definitely 
>> see why. However, the copying the line after that comment is in fact 
>> not atomic.
> Is it assuming "atomicity" by virtue of executing at a safepoint?
> David
> -----
>> Here is the copying code in templateInterpreter.cpp:
>> static inline void copy_table(address* from, address* to, int size) {
>>    // Copy non-overlapping tables. The copy has to occur word wise 
>> for MT safety.
>>    while (size-- > 0) *to++ = *from++;
>> }
>> Copying using a loop of non-volatile loads and stores can and 
>> definitely will on some compilers turn into memcpy calls instead as 
>> the compiler (correctly) considers that an equivalent transformation. 
>> And memcpy does not guarantee atomicity. Indeed on some platforms it 
>> is not atomic. On some platforms it will even enjoy out-of-thin-air 
>> values. Perhaps Copy::disjoint_words_atomic() would be a better 
>> choice for atomic word copying. If not, at the very least we should 
>> use Atomic::load/store here.
>> Having said that, the fix for that issue seems like a separate RFE, 
>> because it has been sitting there for a lot longer than TLH has been 
>> around.
>> Thanks,
>> /Erik
>> On 2019-07-04 04:04, Daniel D. Daugherty wrote:
>>> Greetings,
>>> Robbin recently discovered this issue with Thread Local Handshakes. 
>>> Since
>>> he's not available at the moment, I'm handling the issue:
>>>      JDK-8227117 normal interpreter table is not restored after 
>>> single stepping with TLH
>>>      https://bugs.openjdk.java.net/browse/JDK-8227117
>>> When using Thread Local Handshakes, the normal interpreter table is
>>> not restored after single stepping. This issue is caused by the
>>> VM_ChangeSingleStep VM-op relying on SafepointSynchronize::end() to
>>> restore the normal interpreter table for the "off" case.
>>> Prior to Thread Local Handshakes, this was a valid assumption to make.
>>> SafepointSynchronize::end() has been refactored into
>>> disarm_safepoint() and it only calls Interpreter::ignore_safepoints()
>>> on the global safepoint branch. That matches up with the call to
>>> Interpreter::notice_safepoints() that is also on the global safepoint
>>> branch.
>>> The solution is for the VM_ChangeSingleStep VM-op for the "off" case
>>> to call Interpreter::ignore_safepoints() directly.
>>> Here's the webrev URL:
>>> http://cr.openjdk.java.net/~dcubed/8227117-webrev/0_for_jdk14/
>>> The fix is just a small addition to VM_ChangeSingleStep::doit():
>>>     if (_on) {
>>>       Interpreter::notice_safepoints();
>>> +  } else {
>>> +    Interpreter::ignore_safepoints();
>>>     }
>>> Everything else is just new logging support for future debugging of
>>> interpreter table management and single stepping.
>>> Tested this fix with Mach5 Tier[1-3] on the standard Oracle platforms.
>>> Mach5 Tier[4-6] on standard Oracle platforms is running now.
>>> Thanks, in advance, for questions, comments or suggestions.
>>> Dan

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