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

Chris Plummer chris.plummer at oracle.com
Mon Nov 6 18:56:43 UTC 2017

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.



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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