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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-3745) deadlock in SourceSwitchingIterator
Date Thu, 23 Apr 2015 16:57:39 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-3745?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14509355#comment-14509355
] 

Josh Elser commented on ACCUMULO-3745:
--------------------------------------

{code}
-  public synchronized void seek(Range range, Collection<ByteSequence> columnFamilies,
boolean inclusive) throws IOException {
-    this.range = range;
-    this.inclusive = inclusive;
-    this.columnFamilies = columnFamilies;
+  public void seek(Range range, Collection<ByteSequence> columnFamilies, boolean inclusive)
throws IOException {
+    synchronized (copies) {
+      this.range = range;
+      this.inclusive = inclusive;
+      this.columnFamilies = columnFamilies;
{code}

This seems to be drastically different in what's being synchronized. Instead of synchronizing
on this instance (of SSI), synchronizing on copies means that one instance of a SSI will compete
with other instances of SSI for next/seek since we do not make a copy of the List put just
pass the original List. 

{{setInterruptFlag}} looks good though.

As an aside, can you elaborate on why {{seek}} and {{next}} (via {{readNext}}) were synchronized
in the first place? I didn't seem like the implementations were doing anything that was sensitive
to exclusivity (assuming that the underlying iterators are synchronizing themselves as necessary
which may be an incorrect assumption)

> deadlock in SourceSwitchingIterator
> -----------------------------------
>
>                 Key: ACCUMULO-3745
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-3745
>             Project: Accumulo
>          Issue Type: Bug
>          Components: tserver
>    Affects Versions: 1.5.0, 1.5.1, 1.5.2, 1.6.0, 1.6.1, 1.6.2
>         Environment: Large production cluster, with complex iterator trees.
>            Reporter: Eric Newton
>            Priority: Blocker
>             Fix For: 1.5.3, 1.7.0, 1.6.3
>
>         Attachments: ACCUMULO-3745-1.patch, ACCUMULO-3745-2.patch
>
>          Time Spent: 40m
>  Remaining Estimate: 0h
>
> Details come from an offline cluster, so it's difficult to reproduce the exact details.
 A very complex iterator was running over tablet. "deepCopy" may have been called a couple
dozen times, which may have contributed to the problem.
> Relevant facts:
> A scan and a minor compaction created a deadlock which was detected by the java runtime.
> {noformat}
> "Query... ":
>   waiting to lock monitor 0x1234 (object 0x1234, a java.util.Collections$SynchronizedRandomAccessList),

>   which is held by "minor compactor 1"
> "minor compactor 1":
>  waiting to lock monitor 0x9876 (object 0x9876, a org.apache.accumulo.core.iterators.system.SourceSwitchingIterator),

>  which is held by "Query..."
> {noformat}
> Java stacks:
> {noformat}
> "Query..."
>   at java.util.Collections@SynchronizedCollection.add(Collections.java:1636)
>   - waiting to lock <0x1234> (a java.util.Collections$SynchronizedRandomAccessList)
>   at org.apache.accumulo.core.iterators.system.SourceSwitchingIterator.<init>(SourceSwitchingIterator.java:72)
>  at org.apache.accumulo.core.iterators.system.SourceSwitchingIterator.deepCopy(SourceSwitchingIterator:85)
>  - locked <0x9876> (a org.apache.accumulo.core.iterators.system.SourceSwitchingIterator)
>   ... PartialMutationSkippingIterator.deepCopy(InMememoryMap.java:113)
>  ... InMemoryMap#MemoryIterator.deepCopy(InnMemoryMap.java:623)
>  ...
> {noformat}
> and:
> {noformat}
> "minor compactor 1":
>  at org.apache.accumulo.core.iterators.system.SourceSwitchingIterarot._switchNow(SourceSwitchingIterator:171)
>  - waiting to lock <0x9876> (a org.apache.accumulo.core.iterators.system.SourceSwitchingIterator)
>  at org.apache.accumulo.iterators.system.SourceSwitchingIterator.switchNow(SourceSwitchingIterator.java:184)
>  locked <0x1234> (a java.util.Collections#SynhronizedRandomAccessList)
>  at org.apache.accumulo.tserver.InMemoryMap$MemoryIterator.switchNow(InMemoryMap.java:647)
>  ...
> {noformat}



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