RFR(S): 8059334: nsk/jvmti/scenarios/hotswap/HS201/hs201t001 fails with exit code 0 after timeout
dean.long at oracle.com
dean.long at oracle.com
Mon Nov 6 19:14:37 UTC 2017
On 11/6/17 10:56 AM, 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.
> 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.
>> On 11/3/17 5:25 PM, Chris Plummer wrote:
>>> Please review the following:
>>> 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.
>>> 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.
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