RFR(S): 8059334: nsk/jvmti/scenarios/hotswap/HS201/hs201t001 fails with exit code 0 after timeout

Daniel D. Daugherty daniel.daugherty at oracle.com
Mon Nov 6 20:41:23 UTC 2017

On 11/6/17 1:56 PM, Chris Plummer wrote:
> Hi Dean,
> It looks like ciEnv::jvmti_state_changed() is used to support the 
> JVMTI AddCapabilities() interface, which I believe typically a JVMTI 
> agent uses to setup the available capabilities when the agent is first 
> loaded (although capabilities can by changed afterwords also). So I 
> don't see that code as being related to changing the thread to be 
> changed to "interp only" mode.

Agreed. An agent enables a capability to indicate that it might want to
use that capability/feature at some point during the agent's life. The
compiler needs to know if an agent enabled specific capabilities because
it may need to generate different code in order for specific features to
work during compilation.

The "interp only" mode state is set by the VM and is not directly
managed by the agent. The agent may enable capabilities or events
that cause "interp only" mode to be set and cleared, but it is not
a mode that is directly managed by the agent.

Put more simply:

- The agent enables capabilities to indicate what it might want to do.
- The "interp only" mode is set and cleared by the VM as the VM is
   actually doing stuff.


> thanks,
> Chris
> On 11/3/17 9:44 PM, dean.long at oracle.com wrote:
>> I'm not an expert in this area of code, but I'm wondering about 
>> Vladimir's comment about ciEnv::jvmti_state_changed() in the bug 
>> report. With your fix, maybe we don't need to check 
>> ciEnv::jvmti_state_changed() (which doesn't seem to be enough by 
>> itself) and throw away the compiled result.  We could just keep it 
>> around so it can be used when "interp only" mode is switched off.
>> dl
>> On 11/3/17 5:25 PM, Chris Plummer wrote:
>>> Hello,
>>> Please review the following:
>>> https://bugs.openjdk.java.net/browse/JDK-8059334
>>> http://cr.openjdk.java.net/~cjplummer/8059334/webrev.00/webrev.open/
>>> The CR is closed, so I'll try to explain the issue here. The very 
>>> short explanation is that the JVMTI test was enabling SINGLE STEP 
>>> and doing a PopFrame, but the test method managed to get compiled 
>>> and started executing compiled after the thread was put in "interp 
>>> only" mode (which should never happen) and before the PopFrame was 
>>> processed. The cause is a lack of a check for "interp only" mode in 
>>> the OSR related compilation policy code.
>>> Details:
>>> The test is testing JVMTI PopFrame support. The test thread has a 
>>> small method that sits in a tight loop. It will never exit. The main 
>>> thread enables SINGLE STEP on the test thread, and then does a 
>>> PopFrame on the test thread to force it out of the looping method. 
>>> When the test failed due to a time out, I noticed it was still stuck 
>>> in the small method, even though a PopFrame had been requested. 
>>> Further, I noticed the method was compiled, so there was no chance 
>>> the method would ever detect that it should do a PopFrame. Since 
>>> "interp only" mode for SINGLE STEP had been enabled, the method 
>>> should not be running compiled, so clearly something went wrong that 
>>> allowed it to compile and execute.
>>> When SINGLE STEP is requested, JVMTI will deopt the topmost method 
>>> (actually the top 2), put the thread in "interp only" mode, and then 
>>> has checks to make sure the thread continues to execute interpreted. 
>>> To avoid compilation when a back branch tries to trigger one, there 
>>> is a check for "interp only" mode in SimpleThresholdPolicy::event(). 
>>> If the thread is in "interp only" mode, it will prevent compilation. 
>>> SimpleThresholdPolicy::event() is called (indirectly) by 
>>> InterpreterRuntime::frequency_counter_overflow(), which is called 
>>> from the interpreter when the back branch threshold is reached.
>>> After some debugging I noticed when the test timeout happens, 
>>> "interp only" mode is not yet enabled when 
>>> InterpreterRuntime::frequency_counter_overflow() is called, but is 
>>> enabled by the time InterpreterRuntime::frequency_counter_overflow() 
>>> has done the lookup of the nm. So there is a race here allowing the 
>>> thread to begin execution in a compiled method even though "interp 
>>> only" mode is enabled. I think the reason is because we safepoint 
>>> during the compilation, and this allows a SINGLE STEP request to be 
>>> processed, which enables "interp only" mode.
>>> I should add that initially I only saw this bug with -Xcomp, but 
>>> eventually realized it was caused by disabling 
>>> BackgroundCompilation. That makes it much more likely that a SINGLE 
>>> STEP request will come in and be processed during the call to 
>>> InterpreterRuntime::frequency_counter_overflow() (because it will 
>>> block until the compilation completes).
>>> I believe for the fix it is enough just to add an "interp only" mode 
>>> check in InterpreterRuntime::frequency_counter_overflow() after the 
>>> nm lookup, and set it nm to NULL if we are now in "interp only" 
>>> mode. If we are not in "interp only" mode at this point (and start 
>>> executing the compiled method) it should not be possible to enter 
>>> "interp only" mode until we reach a safepoint at some later time, 
>>> and at that point the method will be properly deopt so it can 
>>> execute interpreted.
>>> thanks,
>>> Chris

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