[jira] [Updated] (HBASE-7160) Improve IdLock and remove its minor defects

["Hiroshi Ikeda (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HBASE-7160?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Hiroshi Ikeda updated HBASE-7160:
---------------------------------

    Attachment: HBASE-7160.patch

Added a patch.

I recreated IdLock with only non-blocking methods except for blocking at the locked entry. (And consequently the code might be a little tedious. It might be overkill.)
                
> Improve IdLock and remove its minor defects
> -------------------------------------------
>
>                 Key: HBASE-7160
>                 URL: https://issues.apache.org/jira/browse/HBASE-7160
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Hiroshi Ikeda
>            Priority: Minor
>         Attachments: HBASE-7160.patch
>
>
> Combination of synchronizations and concurrent collections complicates the code, and it is hard to trace the code and to confirm its correctness. We should recreate the class and make it more understandable.
> In the current code, I find the following minor defects:
> (1) In the case that there is a waiting thread for a lock in getLockEntry() and another thread is releasing the lock by calling releaseLockEntry(), trying to get the lock with a 3rd thread by calling getLockEntry() falls into a busy loop until the waiting thread wakes up and gets the lock.
> Even if notify() wakes up the blocked thread and causes a context switch to the waked thread immediately, synchronization might block the waked thread and cause another context switch, and the busy loop might continue for a while.
> (2) In the same case as (1), since releasing the lock is merely notifying without removing the lock-entry from the map, interrupting the waiting thread might leave an unused lock-entry (entry.numWaiters == 0) in the map. This is a memory leak unless the id of the lock is used again.

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