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Posted to dev@jackrabbit.apache.org by "Jukka Zitting (JIRA)" <ji...@apache.org> on 2010/10/19 11:35:29 UTC

[jira] Created: (JCR-2791) Potential deadlock in LockManagerImpl

Potential deadlock in LockManagerImpl
-------------------------------------

                 Key: JCR-2791
                 URL: https://issues.apache.org/jira/browse/JCR-2791
             Project: Jackrabbit Content Repository
          Issue Type: Bug
          Components: jackrabbit-core
            Reporter: Jukka Zitting
            Priority: Minor


I have a deadlock case that consists of the following *three* threads blocking each other:

Thread A - a writer that's waiting for a write lock on the SISM:

    - waiting on <0x29283ca6> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock)
    - locked <0x29283ca6> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock)
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:485)
    at EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock.acquire(Unknown Source)
    at org.apache.jackrabbit.core.state.DefaultISMLocking$WriteLockImpl.<init>(DefaultISMLocking.java:76)
    at org.apache.jackrabbit.core.state.DefaultISMLocking$WriteLockImpl.<init>(DefaultISMLocking.java:70)
    at org.apache.jackrabbit.core.state.DefaultISMLocking.acquireWriteLock(DefaultISMLocking.java:64)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.acquireWriteLock(SharedItemStateManager.java:1471)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.access$200(SharedItemStateManager.java:112)
    at org.apache.jackrabbit.core.state.SharedItemStateManager$Update.begin(SharedItemStateManager.java:555)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.beginUpdate(SharedItemStateManager.java:1110)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.update(SharedItemStateManager.java:1140)
    at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:351)
    at org.apache.jackrabbit.core.state.XAItemStateManager.update(XAItemStateManager.java:354)
    at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:326)
    at org.apache.jackrabbit.core.state.SessionItemStateManager.update(SessionItemStateManager.java:328)
    at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1141)

Thread B - a reader that's waiting for a read lock on the SISM:

    - waiting on <0x7d1d0717> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock)
    - locked <0x7d1d0717> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock)
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:485)
    at EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock.acquire(Unknown Source)
    at org.apache.jackrabbit.core.state.DefaultISMLocking$ReadLockImpl.<init>(DefaultISMLocking.java:102)
    at org.apache.jackrabbit.core.state.DefaultISMLocking$ReadLockImpl.<init>(DefaultISMLocking.java:96)
    at org.apache.jackrabbit.core.state.DefaultISMLocking.acquireReadLock(DefaultISMLocking.java:53)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.acquireReadLock(SharedItemStateManager.java:1457)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.getItemState(SharedItemStateManager.java:250)
    at org.apache.jackrabbit.core.state.LocalItemStateManager.getNodeState(LocalItemStateManager.java:107)
    at org.apache.jackrabbit.core.state.LocalItemStateManager.getItemState(LocalItemStateManager.java:172)
    at org.apache.jackrabbit.core.state.SessionItemStateManager.getItemState(SessionItemStateManager.java:200)
    at org.apache.jackrabbit.core.HierarchyManagerImpl.getItemState(HierarchyManagerImpl.java:152)
    at org.apache.jackrabbit.core.HierarchyManagerImpl.getPath(HierarchyManagerImpl.java:395)
    at org.apache.jackrabbit.core.CachingHierarchyManager.getPath(CachingHierarchyManager.java:232)
    at org.apache.jackrabbit.core.lock.LockManagerImpl.getPath(LockManagerImpl.java:856)
    at org.apache.jackrabbit.core.lock.LockManagerImpl.getLockInfo(LockManagerImpl.java:547)
    at org.apache.jackrabbit.core.lock.XALockManager.isLocked(XALockManager.java:158)
    at org.apache.jackrabbit.core.lock.SessionLockManager.isLocked(SessionLockManager.java:108)
    at org.apache.jackrabbit.core.NodeImpl.isLocked(NodeImpl.java:3325)

Thread C - a writer that's delivering synchronous observation events to a LockManagerImpl instance:

    - waiting on <0x3fc267ca> (a org.apache.jackrabbit.core.state.LocalItemStateManager)
    - locked <0x3fc267ca> (a org.apache.jackrabbit.core.state.LocalItemStateManager)
     owned by Thread B
    at org.apache.jackrabbit.core.state.LocalItemStateManager.getItemState(LocalItemStateManager.java:167)
    at org.apache.jackrabbit.core.state.SessionItemStateManager.getItemState(SessionItemStateManager.java:200)
    at org.apache.jackrabbit.core.ItemManager.getItemData(ItemManager.java:390)
    at org.apache.jackrabbit.core.ItemManager.getItem(ItemManager.java:336)
    at org.apache.jackrabbit.core.ItemManager.getItem(ItemManager.java:615)
    at org.apache.jackrabbit.core.lock.LockManagerImpl.refresh(LockManagerImpl.java:1186)
    at org.apache.jackrabbit.core.lock.LockManagerImpl.nodeAdded(LockManagerImpl.java:1217)
    at org.apache.jackrabbit.core.lock.LockManagerImpl.onEvent(LockManagerImpl.java:1120)
    at org.apache.jackrabbit.core.observation.EventConsumer.consumeEvents(EventConsumer.java:246)
    at org.apache.jackrabbit.core.observation.ObservationDispatcher.dispatchEvents(ObservationDispatcher.java:214)
    at org.apache.jackrabbit.core.observation.EventStateCollection.dispatch(EventStateCollection.java:475)
    at org.apache.jackrabbit.core.state.SharedItemStateManager$Update.end(SharedItemStateManager.java:765)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.update(SharedItemStateManager.java:1140)
    at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:351)
    at org.apache.jackrabbit.core.state.XAItemStateManager.update(XAItemStateManager.java:354)
    at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:326)
    at org.apache.jackrabbit.core.state.SessionItemStateManager.update(SessionItemStateManager.java:328)
    at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1141)

The problem here is that thread C is trying to deliver synchronous observation events (i.e. it's still holding a downgraded SISM lock) to the LockManagerImpl instance of the session used in thread B. The event handling code in LockManagerImpl uses the session-local LocalItemStateManager instance, whose monitor has already been entered by thread B. Thread B is being blocked by thread A and the writer preference nature of the SISM lock, and thread A can't proceed until thread C has released the SISM lock it's currently holding.

The specific LocalItemStateManager synchronization block seen here has already been removed as a part of my JCR-2699 changes, so this specific deadlock can no longer occur with the latest trunk, but I'm still filing this issue at "Minor" priority as we should still review the above scenario for other potential deadlock issues. Ideally we'd either make the LockManagerImpl use normal instead of synchronous observation, or avoid ItemManager access in the LockManagerImpl event processing code.

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[jira] Updated: (JCR-2791) Potential deadlock in LockManagerImpl

Posted by "Jukka Zitting (JIRA)" <ji...@apache.org>.
     [ https://issues.apache.org/jira/browse/JCR-2791?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Jukka Zitting updated JCR-2791:
-------------------------------

    Affects Version/s: 2.0.0
                       2.1.0
                       2.1.1

Note, the above deadlock can occur with Jackrabbit 2.1 or earlier.

> Potential deadlock in LockManagerImpl
> -------------------------------------
>
>                 Key: JCR-2791
>                 URL: https://issues.apache.org/jira/browse/JCR-2791
>             Project: Jackrabbit Content Repository
>          Issue Type: Bug
>          Components: jackrabbit-core
>    Affects Versions: 2.0.0, 2.1.0, 2.1.1
>            Reporter: Jukka Zitting
>            Priority: Minor
>
> I have a deadlock case that consists of the following *three* threads blocking each other:
> Thread A - a writer that's waiting for a write lock on the SISM:
>     - waiting on <0x29283ca6> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock)
>     - locked <0x29283ca6> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock)
>     at java.lang.Object.wait(Native Method)
>     at java.lang.Object.wait(Object.java:485)
>     at EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock.acquire(Unknown Source)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$WriteLockImpl.<init>(DefaultISMLocking.java:76)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$WriteLockImpl.<init>(DefaultISMLocking.java:70)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking.acquireWriteLock(DefaultISMLocking.java:64)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.acquireWriteLock(SharedItemStateManager.java:1471)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.access$200(SharedItemStateManager.java:112)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager$Update.begin(SharedItemStateManager.java:555)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.beginUpdate(SharedItemStateManager.java:1110)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.update(SharedItemStateManager.java:1140)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:351)
>     at org.apache.jackrabbit.core.state.XAItemStateManager.update(XAItemStateManager.java:354)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:326)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.update(SessionItemStateManager.java:328)
>     at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1141)
> Thread B - a reader that's waiting for a read lock on the SISM:
>     - waiting on <0x7d1d0717> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock)
>     - locked <0x7d1d0717> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock)
>     at java.lang.Object.wait(Native Method)
>     at java.lang.Object.wait(Object.java:485)
>     at EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock.acquire(Unknown Source)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$ReadLockImpl.<init>(DefaultISMLocking.java:102)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$ReadLockImpl.<init>(DefaultISMLocking.java:96)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking.acquireReadLock(DefaultISMLocking.java:53)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.acquireReadLock(SharedItemStateManager.java:1457)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.getItemState(SharedItemStateManager.java:250)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.getNodeState(LocalItemStateManager.java:107)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.getItemState(LocalItemStateManager.java:172)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.getItemState(SessionItemStateManager.java:200)
>     at org.apache.jackrabbit.core.HierarchyManagerImpl.getItemState(HierarchyManagerImpl.java:152)
>     at org.apache.jackrabbit.core.HierarchyManagerImpl.getPath(HierarchyManagerImpl.java:395)
>     at org.apache.jackrabbit.core.CachingHierarchyManager.getPath(CachingHierarchyManager.java:232)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.getPath(LockManagerImpl.java:856)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.getLockInfo(LockManagerImpl.java:547)
>     at org.apache.jackrabbit.core.lock.XALockManager.isLocked(XALockManager.java:158)
>     at org.apache.jackrabbit.core.lock.SessionLockManager.isLocked(SessionLockManager.java:108)
>     at org.apache.jackrabbit.core.NodeImpl.isLocked(NodeImpl.java:3325)
> Thread C - a writer that's delivering synchronous observation events to a LockManagerImpl instance:
>     - waiting on <0x3fc267ca> (a org.apache.jackrabbit.core.state.LocalItemStateManager)
>     - locked <0x3fc267ca> (a org.apache.jackrabbit.core.state.LocalItemStateManager)
>      owned by Thread B
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.getItemState(LocalItemStateManager.java:167)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.getItemState(SessionItemStateManager.java:200)
>     at org.apache.jackrabbit.core.ItemManager.getItemData(ItemManager.java:390)
>     at org.apache.jackrabbit.core.ItemManager.getItem(ItemManager.java:336)
>     at org.apache.jackrabbit.core.ItemManager.getItem(ItemManager.java:615)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.refresh(LockManagerImpl.java:1186)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.nodeAdded(LockManagerImpl.java:1217)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.onEvent(LockManagerImpl.java:1120)
>     at org.apache.jackrabbit.core.observation.EventConsumer.consumeEvents(EventConsumer.java:246)
>     at org.apache.jackrabbit.core.observation.ObservationDispatcher.dispatchEvents(ObservationDispatcher.java:214)
>     at org.apache.jackrabbit.core.observation.EventStateCollection.dispatch(EventStateCollection.java:475)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager$Update.end(SharedItemStateManager.java:765)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.update(SharedItemStateManager.java:1140)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:351)
>     at org.apache.jackrabbit.core.state.XAItemStateManager.update(XAItemStateManager.java:354)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:326)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.update(SessionItemStateManager.java:328)
>     at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1141)
> The problem here is that thread C is trying to deliver synchronous observation events (i.e. it's still holding a downgraded SISM lock) to the LockManagerImpl instance of the session used in thread B. The event handling code in LockManagerImpl uses the session-local LocalItemStateManager instance, whose monitor has already been entered by thread B. Thread B is being blocked by thread A and the writer preference nature of the SISM lock, and thread A can't proceed until thread C has released the SISM lock it's currently holding.
> The specific LocalItemStateManager synchronization block seen here has already been removed as a part of my JCR-2699 changes, so this specific deadlock can no longer occur with the latest trunk, but I'm still filing this issue at "Minor" priority as we should still review the above scenario for other potential deadlock issues. Ideally we'd either make the LockManagerImpl use normal instead of synchronous observation, or avoid ItemManager access in the LockManagerImpl event processing code.

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[jira] Commented: (JCR-2791) Potential deadlock in LockManagerImpl

Posted by "Marcel Reutegger (JIRA)" <ji...@apache.org>.
    [ https://issues.apache.org/jira/browse/JCR-2791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12925420#action_12925420 ] 

Marcel Reutegger commented on JCR-2791:
---------------------------------------

Can we close this issue, now that changes for JCR-2699 have been backported to 2.0 and 2.1 branch?

> Potential deadlock in LockManagerImpl
> -------------------------------------
>
>                 Key: JCR-2791
>                 URL: https://issues.apache.org/jira/browse/JCR-2791
>             Project: Jackrabbit Content Repository
>          Issue Type: Bug
>          Components: jackrabbit-core
>    Affects Versions: 2.0.0, 2.1.0, 2.1.1
>            Reporter: Jukka Zitting
>            Priority: Minor
>
> I have a deadlock case that consists of the following *three* threads blocking each other:
> Thread A - a writer that's waiting for a write lock on the SISM:
>     - waiting on <0x29283ca6> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock)
>     - locked <0x29283ca6> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock)
>     at java.lang.Object.wait(Native Method)
>     at java.lang.Object.wait(Object.java:485)
>     at EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$WriterLock.acquire(Unknown Source)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$WriteLockImpl.<init>(DefaultISMLocking.java:76)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$WriteLockImpl.<init>(DefaultISMLocking.java:70)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking.acquireWriteLock(DefaultISMLocking.java:64)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.acquireWriteLock(SharedItemStateManager.java:1471)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.access$200(SharedItemStateManager.java:112)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager$Update.begin(SharedItemStateManager.java:555)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.beginUpdate(SharedItemStateManager.java:1110)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.update(SharedItemStateManager.java:1140)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:351)
>     at org.apache.jackrabbit.core.state.XAItemStateManager.update(XAItemStateManager.java:354)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:326)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.update(SessionItemStateManager.java:328)
>     at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1141)
> Thread B - a reader that's waiting for a read lock on the SISM:
>     - waiting on <0x7d1d0717> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock)
>     - locked <0x7d1d0717> (a EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock)
>     at java.lang.Object.wait(Native Method)
>     at java.lang.Object.wait(Object.java:485)
>     at EDU.oswego.cs.dl.util.concurrent.WriterPreferenceReadWriteLock$ReaderLock.acquire(Unknown Source)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$ReadLockImpl.<init>(DefaultISMLocking.java:102)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking$ReadLockImpl.<init>(DefaultISMLocking.java:96)
>     at org.apache.jackrabbit.core.state.DefaultISMLocking.acquireReadLock(DefaultISMLocking.java:53)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.acquireReadLock(SharedItemStateManager.java:1457)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.getItemState(SharedItemStateManager.java:250)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.getNodeState(LocalItemStateManager.java:107)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.getItemState(LocalItemStateManager.java:172)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.getItemState(SessionItemStateManager.java:200)
>     at org.apache.jackrabbit.core.HierarchyManagerImpl.getItemState(HierarchyManagerImpl.java:152)
>     at org.apache.jackrabbit.core.HierarchyManagerImpl.getPath(HierarchyManagerImpl.java:395)
>     at org.apache.jackrabbit.core.CachingHierarchyManager.getPath(CachingHierarchyManager.java:232)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.getPath(LockManagerImpl.java:856)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.getLockInfo(LockManagerImpl.java:547)
>     at org.apache.jackrabbit.core.lock.XALockManager.isLocked(XALockManager.java:158)
>     at org.apache.jackrabbit.core.lock.SessionLockManager.isLocked(SessionLockManager.java:108)
>     at org.apache.jackrabbit.core.NodeImpl.isLocked(NodeImpl.java:3325)
> Thread C - a writer that's delivering synchronous observation events to a LockManagerImpl instance:
>     - waiting on <0x3fc267ca> (a org.apache.jackrabbit.core.state.LocalItemStateManager)
>     - locked <0x3fc267ca> (a org.apache.jackrabbit.core.state.LocalItemStateManager)
>      owned by Thread B
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.getItemState(LocalItemStateManager.java:167)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.getItemState(SessionItemStateManager.java:200)
>     at org.apache.jackrabbit.core.ItemManager.getItemData(ItemManager.java:390)
>     at org.apache.jackrabbit.core.ItemManager.getItem(ItemManager.java:336)
>     at org.apache.jackrabbit.core.ItemManager.getItem(ItemManager.java:615)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.refresh(LockManagerImpl.java:1186)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.nodeAdded(LockManagerImpl.java:1217)
>     at org.apache.jackrabbit.core.lock.LockManagerImpl.onEvent(LockManagerImpl.java:1120)
>     at org.apache.jackrabbit.core.observation.EventConsumer.consumeEvents(EventConsumer.java:246)
>     at org.apache.jackrabbit.core.observation.ObservationDispatcher.dispatchEvents(ObservationDispatcher.java:214)
>     at org.apache.jackrabbit.core.observation.EventStateCollection.dispatch(EventStateCollection.java:475)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager$Update.end(SharedItemStateManager.java:765)
>     at org.apache.jackrabbit.core.state.SharedItemStateManager.update(SharedItemStateManager.java:1140)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:351)
>     at org.apache.jackrabbit.core.state.XAItemStateManager.update(XAItemStateManager.java:354)
>     at org.apache.jackrabbit.core.state.LocalItemStateManager.update(LocalItemStateManager.java:326)
>     at org.apache.jackrabbit.core.state.SessionItemStateManager.update(SessionItemStateManager.java:328)
>     at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1141)
> The problem here is that thread C is trying to deliver synchronous observation events (i.e. it's still holding a downgraded SISM lock) to the LockManagerImpl instance of the session used in thread B. The event handling code in LockManagerImpl uses the session-local LocalItemStateManager instance, whose monitor has already been entered by thread B. Thread B is being blocked by thread A and the writer preference nature of the SISM lock, and thread A can't proceed until thread C has released the SISM lock it's currently holding.
> The specific LocalItemStateManager synchronization block seen here has already been removed as a part of my JCR-2699 changes, so this specific deadlock can no longer occur with the latest trunk, but I'm still filing this issue at "Minor" priority as we should still review the above scenario for other potential deadlock issues. Ideally we'd either make the LockManagerImpl use normal instead of synchronous observation, or avoid ItemManager access in the LockManagerImpl event processing code.

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