RFR: 8273790: Potential cyclic dependencies between Gregorian and CalendarSystem

Naoto Sato naoto at openjdk.java.net
Fri Sep 24 18:00:52 UTC 2021

On Fri, 24 Sep 2021 14:36:07 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

> Can I please get a review for this change which proposes to fix the issue reported in https://bugs.openjdk.java.net/browse/JDK-8273790?
> As noted in that issue, trying to class load `sun.util.calendar.CalendarSystem` and `sun.util.calendar.Gregorian` concurrently in separate threads can lead to a deadlock because of the cyclic nature of the code in their static initialization. More specifically, consider this case:
>  - Thread T1 initiates a classload on the `sun.util.calendar.CalendarSystem`.
>  - This gives T1 the implicit class init lock on `CalendarSystem`. 
>  - Consider thread T2 which at the same time initiates a classload on `sun.util.calendar.Gregorian` class.
>  - This gives T2 a implicit class init lock on `Gregorian`.
>  - T1, still holding a lock on `CalendarSystem` attempts to load `Gregorian` since it wants to create a (singleton) instance of `Gregorian` and assign it to the `static final GREGORIAN_INSTANCE` member. Since T2 is holding a class init lock on `Gregorian`, T1 ends up waiting
>  - T2 on the other hand is still loading the `Gregorian` class. `Gregorian` itself "is a" `CalendarSystem`, so during this loading of `Gregorian` class, T2 starts travelling up the class hierarchy and asks for a lock on `CalendarSystem`. However T1 is holding this lock and as a result T2 ends up waiting on T1 which is waiting on T2. That triggers this deadlock.
> The linked JBS issue has a thread dump which shows this in action.
> The commit here delays the instance creation of `Gregorian` by moving that instance creation logic from the static initialization of the `CalendarSystem` class, to the first call to `CalendarSystem#getGregorianCalendar()`. This prevents the `CalendarSystem` from needing a lock on `Gregorian` during its static init (of course, unless some code in this static init flow calls `CalendarSystem#getGregorianCalendar()`, in which case it is back to square one. I have verified, both manually and through the jtreg test, that the code in question doesn't have such calls)
> A new jtreg test has been introduced to reproduce the issue and verify the fix. The test in addition to loading these 2 classes in question, also additionally loads a few other classes concurrently. These classes have specific static initialization which leads the calls to `CalendarSystem#getGregorianCalendar()` or `CalendarSystem#forName()`. Including these classes in the tests ensures that this deadlock hasn't "moved" to a different location. I have run multiple runs (approximately 25) of this test with the fix and I haven't seen it deadlock anymore.

Thanks, Jaikiran. Looks good. Some minor comments.

src/java.base/share/classes/sun/util/calendar/CalendarSystem.java line 123:

> 121:      */
> 122:     public static Gregorian getGregorianCalendar() {
> 123:         var gCal = GREGORIAN_INSTANCE;

Do we need the local variable `gCal`?

test/jdk/sun/util/calendar/CalendarSystemDeadLockTest.java line 43:

> 41:  * @run main/othervm CalendarSystemDeadLockTest
> 42:  * @run main/othervm CalendarSystemDeadLockTest
> 43:  * @run main/othervm CalendarSystemDeadLockTest

Just curious, before the fix, how many test instances caused the deadlock? I'd think these 5 runs are arbitrary numbers, Just wanted to have those 5 runs are appropriate.

test/jdk/sun/util/calendar/CalendarSystemDeadLockTest.java line 75:

> 73:         tasks.add(new GetGregorianCalTask(taskTriggerLatch));
> 74:         tasks.add(new GetGregorianCalTask(taskTriggerLatch));
> 75:         final ExecutorService executor = Executors.newFixedThreadPool(tasks.size());

Asserting `tasks.size() == numTasks` may help here.

test/jdk/sun/util/calendar/CalendarSystemDeadLockTest.java line 171:

> 169:         }
> 170:     }
> 171: }

Need a new line at the EOF.


PR: https://git.openjdk.java.net/jdk/pull/5683

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