code review round 0 for ObjectMonitor-JVM/TI hang fix (8028073) (original) (raw)

David Holmes david.holmes at oracle.com
Sun Feb 9 19:37:11 PST 2014

trimming content ...

On 8/02/2014 9:45 AM, Daniel D. Daugherty wrote:

On 2/7/14 2:56 PM, Karen Kinnear wrote:

3. Did I read the code correctly that the Thread::SpinAcquire can make a timed park call on the same thread's ParkEvent? And that this is used to get on and off the wait queue, i.e. to acquire the WaitSetLock? Is there the same risk that a notify might be eaten here also? As far as I can see, Thread::SpinAcquire() does not use a ParkEvent

It sure does:

void Thread::SpinAcquire (volatile int * adr, const char * LockName) { if (Atomic::cmpxchg (1, adr, 0) == 0) { return ; // normal fast-path return }

// Slow-path : We've encountered contention -- Spin/Yield/Block strategy. TEVENT (SpinAcquire - ctx) ; int ctr = 0 ; int Yields = 0 ; for (;;) { while (*adr != 0) { ++ctr ; if ((ctr & 0xFFF) == 0 || !os::is_MP()) { if (Yields > 5) { // Consider using a simple NakedSleep() instead. // Then SpinAcquire could be called by non-JVM threads Thread::current()->_ParkEvent->park(1) ;

So considering Karen's question ... I can't tell for certain. :(

I do not think the SpinAcquire on grabbing the wait-set lock to add to the wait-set can be an issue because we will only park in response to the actual wait, and hence only get unparked due to a notify/notifyAll, but at this point we still own the monitor so no notify/notifyAll is possible.

However, for the removal from the wait-set a more complex analysis is needed. To do the SpinAcquire we must still be flagged as TS_WAIT - which means we have not been notified, but must be returning due to a timeout (or spurious wakeup?). In such circumstances could we be _succ? I don't think so but I'll leave it to Dan to confirm that part :)

David

at all. However, Thread::muxAcquire() does use a ParkEvent, but it is a different ParkEvent. From src/share/vm/runtime/thread.hpp:

ParkEvent * ParkEvent ; // for synchronized() ParkEvent * SleepEvent ; // for Thread.sleep ParkEvent * MutexEvent ; // for native internal Mutex/Monitor ParkEvent * MuxEvent ; // for low-level muxAcquire-muxRelease So ObjectMonitor uses the ParkEvent field and Thread::muxAcquire() uses the MuxEvent. There are some comments in thread.cpp about how MuxEvent could be eliminated and ParkEvent shared, but I don't think we ever want to go there. I also filed this RFE: 8033399 add a separate ParkEvent for JVM/TI RawMonitor use https://bugs.openjdk.java.net/browse/JDK-8033399 just in case the Serviceability team wants to migrate JVM/TI RawMonitors to a separate ParkEvent. Please let me know if you concur that I've resolved issue #3.

If so, I wonder if we want this added unpark to not just be called if JVMTIEVENTMONITORWAITED is enabled? I don't think we need it, but I've noted its removal as a risk. Again, thanks for the review! Dan

thanks, Karen On Feb 1, 2014, at 1:38 PM, Daniel D. Daugherty wrote: Greetings, I have a fix ready for the following bug: 8028073 race condition in ObjectMonitor implementation causing deadlocks https://bugs.openjdk.java.net/browse/JDK-8028073 On the surface, this is a very simple fix that relocates a few lines of code, relocates and rewrites the comments associated with that code and adds several new comments. Of course, in reality, the issue is much more complicated, but I'm hoping to make it easy for anyone not acquainted with this issue to understand what's going on. Here are the JDK9 webrev URLs: OpenJDK: http://cr.openjdk.java.net/~dcubed/8028073-webrev/0-jdk9-hs-runtime/ Oracle internal: http://javaweb.us.oracle.com/~ddaugher/8028073-webrev/0-jdk9-hs-runtime/ The simple summary: - since Java Monitors and JVM/TI RawMonitors share a ParkEvent, it is possible for a JVM/TI monitor event handler to accidentally consume a ParkEvent.unpark() call meant for Java Monitor layer - the original code fix was made on 2005.07.04 using this bug ID: https://bugs.openjdk.java.net/browse/JDK-5030359 - it's the right fix, but it's in the wrong place - the fix needs to be after the JVMTIEVENTMONITORWAITED event handler is called because it is that event handler that can cause the hang

Testing ------- - a new StessMonitorWait test has been created that reliably reproduces the hang in JDK[6789]; see the bug's gory details for the specific versions where the hang has been reproduced - the test reliably reproduces the hang in 5 seconds on my T7600 running Solaris 10u11 X86; 1 minute runs reproduce the hang reliably on other machines - 12 hour stress run of the new test on Linux-X64, MacOS X-X64, Solaris-SPARCV9, Solaris-X64, and Win7-X86 with the JPRT bits did not reproduce the hang - JPRT test job - VM/SQE Adhoc test job on Server VM, fastdebug bits on Linux-X86, Linux-X64, MacOS X-X64, Solaris-SPARCV9, Solaris-X64, Windows-X86, and Windows-X64: - vm.quick - Kitchensink (bigapps) - Weblogic+medrec (bigapps) - runThese (bigapps) The Gory Details Start Here --------------------------- This is the old location of block of code that's being moved: src/share/vm/runtime/objectMonitor.cpp: 1440 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1499 exit (true, Self) ; // exit the monitor 1513 if (node.notified != 0 && succ == Self) { 1514 node.event->unpark(); 1515 } This is the new location of block of code that's being moved: src/share/vm/runtime/objectMonitor.cpp: 1452 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1601 if (JvmtiExport::shouldpostmonitorwaited()) { 1602 JvmtiExport::postmonitorwaited(jt, this, ret == OSTIMEOUT); 1604 if (node.notified != 0 && succ == Self) { 1620 node.event->unpark(); 1621 } The Risks --------- - The code now executes only when the JVMTIEVENTMONITORWAITED event is enabled: - previously it was always executed - while the old code was not effective for the hang that is being fixed with this bug, it is possible that the old code prevented a different bug in the successor protocol from manifesting - thorough analysis of the successor protocol did not reveal a case where the old code was needed in the old location - Thorough analysis indicates that the other JVM/TI monitor events do not need a fix like the one for JVMTIEVENTMONITORWAITED: - the successor protocol is complicated and the analysis could be wrong when certain options are used - comments were added to each location where a JVM/TI monitor event handler is called documenting why a fix like this one is not needed there - if the analysis is wrong, the new comments show where a new code change would be needed The Scenario ------------ I've created a scenario that reproduces this hang: T1 - enters monitor and calls monitor.wait() T2 - enters the monitor, calls monitor.notify() and exits the monitor T3 - enters and exits the monitor T4 - enters the monitor, delays for 5 seconds, exits the monitor A JVM/TI agent that enables JVMTIEVENTMONITORWAITED and has a handler that: enters a raw monitor, waits for 1ms, exits a raw monitor. Here are the six events necessary to make this hang happen: // KEY-EVENT-1a: After being unparked(), T1 has cleared the succ field, but // KEY-EVENT-1b: T3 is exiting the monitor and makes T1 the successor again. // KEY-EVENT-2a: The unpark() done by T3 when it made T1 the successor // KEY-EVENT-2b: is consumed by the JVM/TI event handler. // KEY-EVENT-3a: T3 made T1 the successor // KEY-EVENT-3b: but before T1 could reenter the monitor T4 grabbed it. // KEY-EVENT-4a: T1's TrySpin() call sees T4 as NotRunnable so // KEY-EVENT-4b: T1 bails from TrySpin without touching succ. // KEY-EVENT-5a: T4 sees that T1 is still the successor so // KEY-EVENT-5b: T4 takes the quick exit path (no ExitEpilog) // KEY-EVENT-6a: T1 is about to park and it is the successor, but // KEY-EVENT-6b: T3's unpark has been eaten by the JVM/TI event handler // KEY-EVENT-6c: and T4 took the quick exit path. T1 is about to be stuck. This bug is intertwined with: - The ObjectMonitor successor protocol - the sharing of a ParkEvent between Java Monitors and JVM/TI RawMonitors There is a very long successor.notes attachment to JDK-8028073 that attempts to describe the ObjectMonitor successor protocol. It's good for putting pretty much anyone to sleep. Since this hang reproduces back to JDK6, this bug is taking the easily backported solution of moving the original fix to the right location. The following new bug has been filed for possible future work in this area by the Serviceability Team: 8033399 add a separate ParkEvent for JVM/TI RawMonitor use https://bugs.openjdk.java.net/browse/JDK-8033399 The Symptoms ------------ With intermittent hangs like this, it is useful to know what to look for in order to determine if you are running into this issue: - if you aren't using a debugger or a profiler or some other JVM/TI agent, then this hang is not the same as yours - if your JVM/TI agent isn't using a JVMTIEVENTMONITORWAITED event handler, then this hang is not the same as yours - if your JVMTIEVENTMONITORWAITED event handler is not using JVM/TI RawMonitors, then this hang is not the same as yours - if your JVMTIEVENTMONITORWAITED event handler is calling back into Java code, then you might just be insane and this hang might be similar to yours. However, using a Java callback in an event handler is an even bigger problem/risk so fix that first. - if you one or more threads blocked like this and making no progress, then this hang might be the same as yours: "T1" #22 prio=5 osprio=64 tid=0x00000000009ca800 nid=0x2f waiting for monitor e ntry [0xfffffd7fc0231000] java.lang.Thread.State: BLOCKED (on object monitor) JavaThread state: threadblocked Thread: 0x00000000009ca800 [0x2f] State: atsafepoint hascalledback 0 atp ollsafepoint 0 JavaThread state: threadblocked at java.lang.Object.wait(Native Method) - waiting on <0xfffffd7e6a2b6ff0> (a java.lang.String) at java.lang.Object.wait(Object.java:502) at SMWWorkerThread.run(StressMonitorWait.java:103) - locked <0xfffffd7e6a2b6ff0> (a java.lang.String) "T2" #23 prio=5 osprio=64 tid=0x00000000009cc000 nid=0x30 waiting for monitor e ntry [0xfffffd7fc0130000] java.lang.Thread.State: BLOCKED (on object monitor) JavaThread state: threadblocked Thread: 0x00000000009cc000 [0x30] State: atsafepoint hascalledback 0 atp ollsafepoint 0 JavaThread state: threadblocked at SMWWorkerThread.run(StressMonitorWait.java:120) - waiting to lock <0xfffffd7e6a2b6ff0> (a java.lang.String) "T3" #24 prio=5 osprio=64 tid=0x00000000009ce000 nid=0x31 waiting for monitor e ntry [0xfffffd7fc002f000] java.lang.Thread.State: BLOCKED (on object monitor) JavaThread state: threadblocked Thread: 0x00000000009ce000 [0x31] State: atsafepoint hascalledback 0 atp ollsafepoint 0 JavaThread state: threadblocked at SMWWorkerThread.run(StressMonitorWait.java:139) - waiting to lock <0xfffffd7e6a2b6ff0> (a java.lang.String) Key symptoms in thread T1: - had the object locked: locked <0xfffffd7e6a2b6ff0> (a java.lang.String) - did an Object.wait(): waiting on <0xfffffd7e6a2b6ff0> (a java.lang.String) - is blocked on reentry: waiting for monitor entry [0xfffffd7fc0231000] Key symtoms in thread T2: - is blocked waiting to lock the object: waiting for monitor entry [0xfffffd7fc0130000] waiting to lock <0xfffffd7e6a2b6ff0> (a java.lang.String) Key symtoms in thread T3: - is blocked waiting to lock the object: waiting for monitor entry [0xfffffd7fc002f000] waiting to lock <0xfffffd7e6a2b6ff0> (a java.lang.String)

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