[concurrency-interest] On A Formal Definition of 'Data-Race'

thurstonn thurston at nomagicsoftware.com
Mon Apr 15 12:55:59 EDT 2013

oleksandr otenko wrote
> When "data race" means "broken logic", there must be a place where that 
> logic is defined.

Yes, there is an assumption in my OP that "data-race" ==> "incorrect" (or
"broken logic" in your words)
Vitaly does not necessarily equate "raciness" with "incorrectness" (and
probably Brian as well) and that's OK with me

oleksandr otenko wrote
> Literature on linearizability introduces a history of events (not the 
> only place where it is done so). If a valid reordering of events 
> produces a invalid history, you have a data race. But you need a 
> definition of a valid history.

Yes, what I'm in (a quixotic?) search for is a process that does the
given a set of operations that can execute across multiple threads (like the
example in the OP)
-define a set of "execution histories" (your "history of events") that are
possible given the MM in effect and consistent with the original program's
set of operations (of course there will be multiple such histories)
-each "execution history" defines at least a partial ordering among
conflicting operations (r/w or w/w on the same shared data item)
-analyze each execution history for "correctness"
if each possible history is correct, then you're good
else add explicit happens-before relations. Repeat

oleksandr otenko wrote
> Your argument is that any reordering of instructions in your example is 
> a valid history. But in order to state that, we would need to see the 
> rest of history. The subsequent use of local will determine the validity 
> of history.
> Alex

Agreed (the example 'program' is simplistic at best).

What my original post described was exactly the kind of process (viz. an
acyclic serialization graph) that I'm in search of, but is applied to
database concurrency control.  The problems are very similar (turning
concurrent executions into serial, partially-ordered ones; operations are
reads/writes of data items), but they are not exactly the same.  I was
wondering if we could use the same techniques (with a serialization graph
==> "execution graph") to analyze the "correctness" of, e.g. non-locking
concurrent algorithms/data structures.


On 12/04/2013 19:55, thurstonn wrote:
> In thinking about whether code is thread-safe or not, one can attempt to
> identify whether it 'contains a data-race'.  If not, you're good.  Else
> you
> need to add an explicit happens-before relationship.
> Which begs the question: what exactly constitutes a 'data-race'?  And here
> I'm interested in something a little more formal than the famed judicial
> judgement of obscenity (I know it when I see it)
> If you do a web search, you unfortunately get quite a few divergent
> definitions, many of which seem to be inconsistent.
> IIRC, the official JMM defines a data-race as any two conflicting
> operations
> from two or more threads on shared data (where at least one of the two
> operations is a write).
> Brian Goetz (in his excellent  article
> <http://www.ibm.com/developerworks/library/j-jtp03304/>  ) defines
> data-race
> thusly:
> "A program is said to have a data race, and therefore not be a "properly
> synchronized" program, when there is a variable that is read by more than
> one thread, written by at least one thread, and the write and the reads
> are
> not ordered by a happens-before relationship."
> But this would mark the following code as a data-race
> int shared = 0
> Thread 1                  Thread 2                 Thread 3
> local = this.shared      this.shared = 10       local = this.shared
> This clearly meets his definition, yet I do not consider this a
> 'data-race'.
> I've always relied on traditional database concurrency control theory (I
> still find the treatise by Bernstein, Hadzilacos, and Goodman to be the
> best), which has a formal definition of 'serializability', viz. that any
> transaction log is 'serializable', if and only if, its serialization graph
> is acyclic.  Why can we not use this as the basis for a formal definition
> of
> 'data-race' (excluding the notion of commit and abort of course):
> "A program is said to have a data-race, if any legal (as prescribed by the
> MM) execution order produces a serialization graph that is *cyclic*"
> It has the advantage of a formal, mathematical model and although it is
> has
> historically been confined to databases (and transactions), it seems
> applicable to concurrent execution of any kind?
> Hoping that I don't get flamed.
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