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

Daniel D. Daugherty daniel.daugherty at oracle.com
Fri Jul 5 17:37:28 UTC 2019

On 7/4/19 4:08 AM, Erik Österlund wrote:
> 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.

It could also be that our older compilers on Solaris weren't making this
transformation prior to TLH and we were less exposed to the possibility
of the race. TLH landed in JDK10 (JDK-8189941). As far I can tell, it
looks like TLH was enabled by default on platforms that could support
it when it landed.


> /Erik
> 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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