Hi Vladimir,
I also considered calling nm->make_not_entrant(), but came to the same
conclusion as you.
thanks,
Chris
On 11/7/17 10:39 AM, Vladimir Kozlov wrote:
Looks good to me too. I assume you will add NULL check as Dan suggested.
I thought that you may need to call nm->make_not_entrant() to
deoptimize method. But on other hand you may still want to run
compiled code in other threads. So your fix should is good for your
problem.
Thanks,
Vladimir
On 11/6/17 4:58 PM, [email protected] wrote:
Hi Chris,
The fix looks good.
I'm not that familiar with the compiler code to judge if this the
best place to make this check.
But, at least, it looks as such to me.
Perhaps, it would be great if Vladimir could also look at it.
But now pressure for this.
Thanks,
Serguei
On 11/3/17 17:25, 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
