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

David Holmes david.holmes at oracle.com
Mon Feb 3 22:39:48 PST 2014


On 4/02/2014 12:56 AM, Daniel D. Daugherty wrote:
> Adding Dave Dice to this thread...
>
> On 2/3/14 5:10 AM, David Holmes wrote:
>> Hi Dan,
>>
>> On 2/02/2014 4:38 AM, 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.
>>
>> Let's spare them the trouble - Reviewed :)
>
> Thanks for the review!
>
>
>> I'm still troubled by the rationale for the original placement of this
>> code. It makes sense to issue the unpark prior to the park() call but
>> on the other hand there is no agent/event code in that path that would
>> have consumed the original unpark. ??
>
> I think I've acknowledged that concern in the "Risks" section below:
>
>     - 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
>
> Hopefully, Dave Dice will chime in and share the history...

For the record Dave did chime in off-list and this code block dates back 
to the JVMPI implementation which did 
jvmpi::post_monitor_contended_exit_event at the end of exit(), which is 
called just before the code block in question. That was back in JDK 5. 
By JDK 6 JVMPI had been stripped out but the code block remained.

>> Thanks, I know you've really enjoyed this one :)
>
> I actually did enjoy this one from a 'challenge to the mind' POV.
> But I have to say that the intricacies of the successor protocol
> gave me headaches... Hopefully, my successor.notes will help anyone
> else that is crazy enough to wander down this path...

Indeed!

Thanks,
David


> Dan
>
>
>>
>> David
>> -----
>>
>>> 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 JVMTI_EVENT_MONITOR_WAITED
>>>    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) {
>>> <snip>
>>> 1499    exit (true, Self) ;                    // exit the monitor
>>> <snip>
>>> 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) {
>>> <snip>
>>> 1601      if (JvmtiExport::should_post_monitor_waited()) {
>>> 1602        JvmtiExport::post_monitor_waited(jt, this, ret ==
>>> OS_TIMEOUT);
>>> <snip>
>>> 1604        if (node._notified != 0 && _succ == Self) {
>>> <snip>
>>> 1620          node._event->unpark();
>>> 1621        }
>>>
>>>
>>> The Risks
>>> ---------
>>>
>>> - The code now executes only when the JVMTI_EVENT_MONITOR_WAITED 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 JVMTI_EVENT_MONITOR_WAITED:
>>>    - 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 JVMTI_EVENT_MONITOR_WAITED 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 JVMTI_EVENT_MONITOR_WAITED
>>>    event handler, then this hang is not the same as yours
>>> - if your JVMTI_EVENT_MONITOR_WAITED event handler is not using
>>>    JVM/TI RawMonitors, then this hang is not the same as yours
>>> - if your JVMTI_EVENT_MONITOR_WAITED 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 os_prio=64 tid=0x00000000009ca800 nid=0x2f waiting for
>>> monitor e
>>> ntry [0xfffffd7fc0231000]
>>>     java.lang.Thread.State: BLOCKED (on object monitor)
>>>     JavaThread state: _thread_blocked
>>> Thread: 0x00000000009ca800  [0x2f] State: _at_safepoint _has_called_back
>>> 0 _at_p
>>> oll_safepoint 0
>>>     JavaThread state: _thread_blocked
>>>          at java.lang.Object.wait(Native Method)
>>>          - waiting on <0xfffffd7e6a2b6ff0> (a java.lang.String)
>>>          at java.lang.Object.wait(Object.java:502)
>>>          at SMW_WorkerThread.run(StressMonitorWait.java:103)
>>>          - locked <0xfffffd7e6a2b6ff0> (a java.lang.String)
>>>
>>> "T2" #23 prio=5 os_prio=64 tid=0x00000000009cc000 nid=0x30 waiting for
>>> monitor e
>>> ntry [0xfffffd7fc0130000]
>>>     java.lang.Thread.State: BLOCKED (on object monitor)
>>>     JavaThread state: _thread_blocked
>>> Thread: 0x00000000009cc000  [0x30] State: _at_safepoint _has_called_back
>>> 0 _at_p
>>> oll_safepoint 0
>>>     JavaThread state: _thread_blocked
>>>          at SMW_WorkerThread.run(StressMonitorWait.java:120)
>>>          - waiting to lock <0xfffffd7e6a2b6ff0> (a java.lang.String)
>>>
>>> "T3" #24 prio=5 os_prio=64 tid=0x00000000009ce000 nid=0x31 waiting for
>>> monitor e
>>> ntry [0xfffffd7fc002f000]
>>>     java.lang.Thread.State: BLOCKED (on object monitor)
>>>     JavaThread state: _thread_blocked
>>> Thread: 0x00000000009ce000  [0x31] State: _at_safepoint _has_called_back
>>> 0 _at_p
>>> oll_safepoint 0
>>>     JavaThread state: _thread_blocked
>>>          at SMW_WorkerThread.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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