[netsa-tools-discuss] segfaults in rwflowpack/rwflowappend

[John Green]

On Wed, 2014-12-03 at 17:43 -0500, Michael Welsh Duggan wrote:
> We found it!  You were correct.  There is an extremely subtle bug
> involving the redblack code and multithreading.  The redblack
> implementation we are using uses a global null element (called RBNULL in
> the code).  This null element, startlingly, is not necessarily
> constant.  When deleting a node, the null element's color and up pointer
> can both change and be used.  If another thread is also deleting an
> element from a tree, they can both be modifying this null element.

Great news.   Ironically as part of my debugging I compiled Silk with
clang + thread sanitizer and it identified exactly the same issue.  It
didn't make much sense to me though (I hadn't spotted that RBNULL was
global) so I parked it whilst I investigated other possibilities.   

Thanks for investigating this.

John

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