[GitHub] merlimat opened a new pull request #1607: Issue 1606: Fixed race condition during expansion of concurrent open hash maps

[GitBox <gi...@apache.org>]

merlimat opened a new pull request #1607: Issue 1606: Fixed race condition during expansion of concurrent open hash maps
URL: https://github.com/apache/bookkeeper/pull/1607
 
 
   ### Motivation
   
   As reported in #1606, there is a race condition in the concurrent open hash maps implementation. The race happens when the maps gets re-hashed after the expansion and the new arrays are substituting the old ones.
   
   The race itself is that a thread doing a `get()` on the map is first checking the current `capacity` of the map, uses that to get the bucket and then tries to do optimistic read of the value in that bucket. 
   
   This assumes `capacity` update is visible only after the `values` array is already swapped, but that is not always the case in current code.
   
   ### Changes
    * Use `volatile` qualifier for `capacity` and `values` arrays to ensure ordering of memory read is respected by compiler
    * In rehashing, update `capacity` after `values`

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