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https://issues.apache.org/jira/browse/HDFS-7495?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14334283#comment-14334283
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Colin Patrick McCabe edited comment on HDFS-7495 at 2/24/15 2:35 AM:
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As you commented, on closer inspection, this code isn't incorrect, it is just
very, very tricky.
The patch you posted is a step in the right direction, but it still modifies
these variables without any synchronization, which will lead to findbugs
warnings (as you saw).
I think this can be refactored to move that block out of getBlockAt altogether
to avoid both findbugs and the confusing code.
was (Author: cmccabe):
[~tedyu], please don't close things without at least an explanation.
On closer inspection, this code isn't incorrect, it is just very, very tricky.
The comment here is the clue:
{code}
// synchronized not strictly needed, since we only get here
// from synchronized caller methods
{code}
The intent is that there is no "real" lock inversion because we only ever call
getBlockAt with updatePosition = true when we already hold the stream lock. Of
course, these are re-entrant locks, like all Java locks The "synchronized"
block is actually probably only there for findbugs.
Although this code is correct, it is not maintainable. It's just too
confusing. I think this can be refactored to avoid this block.
> Lock inversion in DFSInputStream#getBlockAt()
> ---------------------------------------------
>
> Key: HDFS-7495
> URL: https://issues.apache.org/jira/browse/HDFS-7495
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Ted Yu
> Priority: Minor
> Attachments: hdfs-7495-001.patch
>
>
> There're two locks: one on DFSInputStream.this , one on
> DFSInputStream.infoLock
> Normally lock is obtained on infoLock, then on DFSInputStream.infoLock
> However, such order is not observed in DFSInputStream#getBlockAt() :
> {code}
> synchronized(infoLock) {
> ...
> if (updatePosition) {
> // synchronized not strictly needed, since we only get here
> // from synchronized caller methods
> synchronized(this) {
> {code}
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