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

[David Holmes]

Hi Dan,

On 4/07/2019 12:04 pm, 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.

So the result of this is that debugging tests may run more slowly overall?

> 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();
>     }

Looks good - thanks for the detailed analysis in the bug report.

I have on additional request from looking at related code - can you fix 
this confused initializer:

VM_ChangeSingleStep::VM_ChangeSingleStep(bool on)
   : _on(on != 0)
{
}

as _on and on are both bool the assignment can be direct and we 
shouldn't be comparing a bool to 0 as a matter of style. Thanks.

> Everything else is just new logging support for future debugging of
> interpreter table management and single stepping.

Logging looks good too.

Thanks,
David
-----

> 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
>