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https://issues.apache.org/jira/browse/HBASE-20846?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16540097#comment-16540097
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Duo Zhang commented on HBASE-20846:
-----------------------------------

I'm reading the code of WALProcedureStore... It is like a black magic for me, 
just feel the same when I start reading the code of 
MasterProcedureScheduler/ProcedureExecutor... Need some time to understand it...

> Table's shared lock is not held by sub-procedures after master restart
> ----------------------------------------------------------------------
>
>                 Key: HBASE-20846
>                 URL: https://issues.apache.org/jira/browse/HBASE-20846
>             Project: HBase
>          Issue Type: Sub-task
>    Affects Versions: 2.1.0
>            Reporter: Allan Yang
>            Assignee: Allan Yang
>            Priority: Major
>             Fix For: 3.0.0, 2.0.2, 2.1.1
>
>         Attachments: HBASE-20846.branch-2.0.002.patch, 
> HBASE-20846.branch-2.0.patch
>
>
> Found this one when investigating ModifyTableProcedure got stuck while there 
> was a MoveRegionProcedure going on after master restart.
> Though this issue can be solved by HBASE-20752. But I discovered something 
> else.
> Before a MoveRegionProcedure can execute, it will hold the table's shared 
> lock. so,, when a UnassignProcedure was spwaned, it will not check the 
> table's shared lock since it is sure that its parent(MoveRegionProcedure) has 
> aquired the table's lock.
> {code:java}
> // If there is parent procedure, it would have already taken xlock, so no 
> need to take
>       // shared lock here. Otherwise, take shared lock.
>       if (!procedure.hasParent()
>           && waitTableQueueSharedLock(procedure, table) == null) {
>           return true;
>       }
> {code}
> But, it is not the case when Master was restarted. The child 
> procedure(UnassignProcedure) will be executed first after restart. Though it 
> has a parent(MoveRegionProcedure), but apprently the parent didn't hold the 
> table's lock.
> So, since it began to execute without hold the table's shared lock. A 
> ModifyTableProcedure can aquire the table's exclusive lock and execute at the 
> same time. Which is not possible if the master was not restarted.
> This will cause a stuck before HBASE-20752. But since HBASE-20752 has fixed, 
> I wrote a simple UT to repo this case.
> I think we don't have to check the parent for table's shared lock. It is a 
> shared lock, right? I think we can acquire it every time we need it.



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