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From "Tom White (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3455) IPC.Client synchronisation looks weak
Date Wed, 28 May 2008 18:09:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3455?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12600548#action_12600548
] 

Tom White commented on HADOOP-3455:
-----------------------------------

> Doesn't Java guarantee that booleans (& ints) are written and read atomically? Isn't
that sufficient here?

Strictly speaking I don't think it is sufficient. Java guarantees atomicity, but it doesn't
guarantee that a thread will see writes made by another thread unless the field is marked
as volatile, or is accessed in a synchronized block.

> IPC.Client synchronisation looks weak
> -------------------------------------
>
>                 Key: HADOOP-3455
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3455
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: ipc
>    Affects Versions: 0.18.0
>            Reporter: Steve Loughran
>         Attachments: hadoop-3455.patch
>
>
> Looking at HADOOP-3453 , its clear that Client.java is inconsistently synchronized
> 1. the running and shouldCloseConnection flags are not always read/written in synchronized
blocks, even though they are properties used to share information between threads. They should
be marked as volatile for access outside synchronized blocks, and all read-check-update operations
must be synchronized.
> 2. there are multiple calls to System.currentTimeMillis() in synchronized blocks; this
is a slow native operation and should ideally be done unsynchronized.
> 3. Synchronizing on the (out) stream is dangerous as its value changes during the life
of the class, and sometimes it is null. These blocks should all synchronize on the Client
instead.
> 4.  There are a number of places where InterruptedExceptions are caught and ignored in
a sleep-wait loop:
>      } catch (InterruptedException e) {
>       }
>    This isn't dangerous, but it does make the client harder to stop. These code fragments
should be looked at carefully.

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