On 11/12/19 5:50 PM, David Holmes wrote:
Hi Dan,
Thanks for taking a look so quickly!
Your welcome! I figured you would prefer to get this one out of
the way quickly.
On 13/11/2019 3:18 am, Daniel D. Daugherty wrote:
On 11/11/19 11:52 PM, David Holmes wrote:
webrev: http://cr.openjdk.java.net/~dholmes/8233549/webrev/
src/hotspot/os/posix/os_posix.cpp
L2078: // Can't access interrupt state now we are
_thread_blocked. If we've been
L2079: // interrupted since we checked above then _counter
will be > 0.
nit - grammar. Please consider:
// Can't access interrupt state now that we are
_thread_blocked. If we've
// been interrupted since we checked above then
_counter will be > 0.
src/hotspot/os/solaris/os_solaris.cpp
L4924: // Can't access interrupt state now we are
_thread_blocked. If we've been
L4925: // interrupted since we checked above then _counter
will be > 0.
nit - grammar. Please consider:
// Can't access interrupt state now that we are
_thread_blocked. If we've
// been interrupted since we checked above then
_counter will be > 0.
Will fix grammatical nits.
src/hotspot/share/classfile/javaClasses.cpp
No comments.
src/hotspot/share/prims/jvmtiEnv.cpp
Hmmm... did the "non-JavaThread can't be interrupted" check also
get
pushed down?
Update: Similar check is now in JvmtiRawMonitor::raw_wait().
src/hotspot/share/prims/jvmtiRawMonitor.cpp
L239: ThreadInVMfromNative tivm(jt);
L240: if (jt->is_interrupted(true)) {
L241: ret = M_INTERRUPTED;
L242: } else {
L243: ThreadBlockInVM tbivm(jt);
L244: jt->set_suspend_equivalent();
L245: if (millis <= 0) {
L246: self->_ParkEvent->park();
L247: } else {
L248: self->_ParkEvent->park(millis);
L249: }
L250: }
L251: // Return to VM before post-check of interrupt state
L252: if (jt->is_interrupted(true)) {
The comment on L251 is better between L249 and L250 since that
is where 'tbivm' gets destroyed and you transition back.
You could have this comment before L252:
// Must be in VM to safely access interrupt state:
if you think you really need a comment there.
Will move comment up as suggested.
src/hotspot/share/prims/jvmtiRawMonitor.hpp
No comments.
src/hotspot/share/runtime/objectMonitor.cpp
You've moved the is_interrupted() check from after ThreadBlockInVM
to before it. ThreadBlockInVM can block for a safepoint which
widens
the window for an interrupt to come in after the check on L1272 and
and before the thread parks on L1286 or L1288.
Can this result in an unexpected park() where before we would have
taken the "Intentionally empty" code path on L1283?
What I'm worried about is whether we've opened a window where we
do Object.wait(0) and that wait() is supposed to be interrupted.
However, we lose that interrupt because it arrives in the now wider
window between L1272 and L1286 and we never return from the
wait(0).
It is possible that I'm not remembering something about how
interrupt()
interacts with park().
The interrupt() not only sets the field but also issues an unpark() to
the ParkEvent. So if we are interrupted whilst processing through the
TBIVM, the call to park() will return immediately as the ParkEvent
will be in the signalled state.
That was the piece I wasn't remembering. Thanks for filling in the detail.
test/hotspot/jtreg/ProblemList.txt
Thanks for remembering to update the ProblemList.
The only part I'm worried about is ObjectMonitor::wait(). If my worry is
baseless, then thumbs up.
Worry is baseless :)
Agreed!
I have a couple of nits above. If you choose to fix those, then I don't
need to see another webrev.
Thanks again!
You're welcome.
Dan
David
-----
Dan
bug: https://bugs.openjdk.java.net/browse/JDK-8233549
In JDK-8229516 I moved the interrupted state of a thread from the
osThread in the VM to the java.lang.Thread instance. In doing that I
overlooked a critical aspect, which is that to access the field of a
Java object the JavaThread must not be in a safepoint-safe state** -
otherwise the oop, and anything referenced there from could be
relocated by the GC whilst the JavaThread is accessing it. This
manifested in a number of tests using JVM TI Agent threads and JVM
TI RawMonitors because the JavaThread's were marked _thread_blocked
and hence safepoint-safe, and we read a non-zero value for the
interrupted field even though we had never been interrupted.
This problem existed in all the code that checks for interruption
when "waiting":
- Parker::park (the code underpinning
java.util.concurrent.LockSupport.park())
To fix this code I simply deleted a late check of the interrupted
field. The check was not needed because if an interrupt has occurred
then we will find the ParkEvent in a signalled state.
- ObjectMonitor::wait
Here the late check of the interrupted state is essential as we
reset the ParkEvent after an earlier check of the interrupted state.
But the fix was simply achieved by moving the check slightly earlier
before we use ThreadBlockInVm to become _thread_blocked.
- RawMonitor::wait
This fix was much more involved. The RawMonitor code directly
transitions the JavaThread from _thread_in_Native to
_thread_blocked. This is safe from a safepoint perspective because
they are equivalent safepoint-safe states. To allow access to the
interrupted field I have to transition from native to _thread_in_vm,
and that has to be done by proper thread-state transitions to ensure
correct access to the oop and its fields. Having done that I can
then use ThreadBlockInVM for the transitions to blocked. However, as
the old code noted it can't use proper thread-state transitions as
this will lead to deadlocks with the VMThread that can also use
RawMonitors when executing various event callbacks. To deal with
that we have to note that the real constraint is that the JavaThread
cannot block at a safepoint whilst it holds the RawMonitor. Hence
the fix was push all the interrupt checking code and the
thread-state transitions to the lowest level of RawMonitorWait,
around the final park() call, after we have enqueued the waiter and
released the monitor. That avoids any deadlock possibility.
I also added checks to is_interrupted/interrupted to ensure they are
only called by a thread in a suitable state. This should only be the
VMThread (as a consequence of the Thread.stop implementation
occurring at a safepoint and issuing a JavaThread::interrupt() call
to unblock the target); or a JavaThread that is not
_thread_in_native or _thread_blocked.
Testing: (still finalizing)
- tiers 1 - 6 (Oracle platforms)
- Local Linux testing
- vmTestbase/nsk/monitoring/
- vmTestbase/nsk/jdwp
- vmTestbase/nsk/jdb/
- vmTestbase/nsk/jdi/
- vmTestbase/nsk/jvmti/
- serviceability/jvmti/
- serviceability/jdwp
- JDK: java/lang/management
com/sun/management
** Note that this applies to all accesses we make via code in
javaClasses.*. For this particular code I thought about adding a
guard in JavaThread::threadObj() but it turns out when we generate a
crash report we access the Thread's name() field and that can happen
when in any state, so we'd always trigger a secondary assertion
failure during error reporting if we did that. Note that accessing
name() can still easily lead to secondary assertions failures as I
discovered when trying to debug this and print the thread name out -
I would see an is_instance assertion fail checking that the Thread
name() is an instance of java.lang.String!
Thanks,
David
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