[jmm-dev] Sequential Consistency

Paul E. McKenney paulmck at linux.vnet.ibm.com
Tue Feb 25 10:53:08 PST 2014

On Tue, Feb 25, 2014 at 07:26:45AM -0500, Doug Lea wrote:
> On 02/24/2014 05:44 PM, Paul E. McKenney wrote:
> >This is my experience as well -- I have seen very few actual algorithms
> >that relied on SC.
> This seems to be the attitude of almost all developers of
> non-lock-based algorithms: Explicit ordering constraints are
> critical, but program-wide SC is not. Which is nearly opposite
> to almost every developer's view of lock-based programs: any ordering
> is OK so long as SC is maintained.

I am quite capable of maintaining both viewpoints internally.
If I am using locks, I want the benefits of locking.  When I am
not using locks, I don't want to be forced to wear the locking
straightjacket.  ;-)

> One place these different views meet up is when creating locks
> out of non-blocking primitives. So there must be guaranteed
> ways of achieving SC using modeful/fenced accesses.


> Beyond that, the problem seems underconstrained.
> I'm not sure that litmus-test-style examples will suffice
> to provide an answer. When you are not dealing with locks,
> it seems that for every odd consequence of some non-SC rule,
> you can find an equally odd one for an SC-based rule.
> For example, Ali Sezgin (who is on this list) has written up some
> especially bizarre sequentially consistent examples in:
> Sezgin, Ali, and Ganesh Gopalakrishnan. "On the definition of
> sequential consistency." Information processing letters 2005.
> http://www.cs.utah.edu/formal_verification/publications/june2013update/dblp/2005/2/j23.pdf

I had not seen this one before!  Classic!!!  ;-)

							Thanx, Paul

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