[jira] [Resolved] (SLIDER-1242) Review uses of double-checked locking

["Billie Rinaldi (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SLIDER-1242?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Billie Rinaldi resolved SLIDER-1242.
------------------------------------
    Resolution: Fixed

> Review uses of double-checked locking
> -------------------------------------
>
>                 Key: SLIDER-1242
>                 URL: https://issues.apache.org/jira/browse/SLIDER-1242
>             Project: Slider
>          Issue Type: Bug
>            Reporter: Billie Rinaldi
>            Assignee: Billie Rinaldi
>             Fix For: Slider 1.0.0
>
>         Attachments: SLIDER-1242.1.patch
>
>
> There are several places where we perform double-checked locking. Even though the practice is discouraged, I believe it is technically correct when the check is performed on the presence of a ConcurrentHashMap key, which is how we are using it.
> However, in two places, AgentProviderService#getCurrentExports and AgentProviderService#getAllocatedPorts, containsKey is used instead of get to perform the check. I am seeing some indication that containsKey is not sufficient, and that get must be used for double-checked locking to be correct. There is a comment in the ConcurrentHashMap#containsKey method that says "same as get() except no need for volatile value read" -- and I think that volatile value read is what we need for correctness.
> Also, in the [ConcurrentHashMap api doc|https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html], it specifically mentions get and does not mention containsKey: "Any non-null result returned from get(key) and related access methods bears a happens-before relation with the associated insertion or update" and "an update operation for a given key bears a happens-before relation with any (non-null) retrieval for that key reporting the updated value."



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