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From "Michael McCandless (JIRA)" <>
Subject [jira] [Updated] (LUCENE-7570) Tragic events during merges can lead to deadlock
Date Fri, 09 Dec 2016 01:56:58 GMT


Michael McCandless updated LUCENE-7570:
    Attachment: LUCENE-7570.patch

Here's a patch w/ test case reproducing the deadlock, and a simple fix, that just postpones
launching merges until after we are out of the commit lock.

> Tragic events during merges can lead to deadlock
> ------------------------------------------------
>                 Key: LUCENE-7570
>                 URL:
>             Project: Lucene - Core
>          Issue Type: Bug
>          Components: core/index
>    Affects Versions: 5.5, master (7.0)
>            Reporter: Joey Echeverria
>         Attachments: LUCENE-7570.patch, thread_dump.txt
> When an {{IndexWriter#commit()}} is stalled due to too many pending merges, you can get
a deadlock if the currently active merge thread hits a tragic event.
> # The thread performing the commit synchronizes on the the {{commitLock}} in {{commitInternal}}.
> # The thread goes on to to call {{ConcurrentMergeScheduler#doStall()}} which {{waits()}}
on the {{ConcurrentMergeScheduler}} object. This release the merge scheduler's monitor lock,
but not the {{commitLock}} in {{IndexWriter}}.
> # Sometime after this wait begins, the merge thread gets a tragic exception can calls
{{IndexWriter#tragicEvent()}} which in turn calls {{IndexWriter#rollbackInternal()}}.
> # The {{IndexWriter#rollbackInternal()}} synchronizes on the {{commitLock}} which is
still held by the committing thread from (1) above which is waiting on the merge(s) to complete.
Hence, deadlock.
> We hit this bug with Lucene 5.5, but I looked at the code in the master branch and it
looks like the deadlock still exists there as well.

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