full-disclosure - Re: [Code-Crunchers] a simple race condition and how you'd solve it

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Message-ID: <12004.1246586666@turing-police.cc.vt.edu>
Date: Thu, 02 Jul 2009 22:04:26 -0400
From: Valdis.Kletnieks@...edu
To: michaelslists@...il.com
Cc: full-disclosure@...ts.grok.org.uk, fuzzing@...testar.linuxbox.org,
	code-crunchers@...testar.linuxbox.org, Gadi Evron <ge@...uxbox.org>
Subject: Re: [Code-Crunchers] a simple race condition and
	how you'd solve it

On Fri, 03 Jul 2009 11:01:34 +1000, silky said:

> Basically, you just need to check if you should still be computing,
> and, at the end of computation, if your data is still "wanted".

All that does is push the race condition around.  You *still* need to
do some sort of locking around the tail end.  This is still racy:

    if (update_still_wanted) {
	stash_my_update();
        update_still_wanted = false;
    }

(Admittedly, not *as* racy, especially if you move the assignment first. But
that's still racy enough to actually *trip* on occasion - this sort of bug
is actually found at least once a month in the Linux kernel in some device
driver or other...)

And to be honest - the "best" way of fixing this is *really* going to depend on
the relative weight of locking (which can be *very* different if you have 2
threads on 2 CPUs, or 4096 threads on a 4096-core monster, or are split across
systems possibly in different countries connected by a high or maybe low speed
network), and how much effort goes into the computation, and how much
correctness matters - for some cases, you *really* want "first to finish"
(possibly due to side effects of the computation), others "any complete answer"
is good enough, etc..

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