On Thu, Aug 10, 2006 at 10:27:42PM +0200, Stelian Pop wrote:
> [[email protected] bouncing, removed from CC:]
>
> Le jeudi 10 août 2006 à 09:47 -0700, Paul E. McKenney a écrit :
>
> > > Let's take this problem differently: is a memory barrier cheaper than a
> > > spinlock ?
> >
> > Almost always, yes. But a spinlock is cheaper than a spinlock plus
> > a pair of memory barriers.
>
> Right, but I think we're optimizing too much here.
That was in fact my point initially -- why not just require locking,
either that registered at kfifo_alloc() time or a separately acquired
lock?
> > > If the answer is yes as I suspect, why should the kfifo API force the
> > > user to take a spinlock ?
> >
> > My concern is that currently a majority of the calls to __kfifo_{get,put}()
> > are already holding a spinlock.
> >
> > But if you could send me your tests for lock-free __kfifo_{get,put}(),
> > I would be happy to run them on weak-memory-consistency model machines
> > with the memory barriers. And without the memory barriers -- we need
> > a test that fails in the latter case to prove that the memory barriers
> > really are in the right place and that all of them are present.
> >
> > Does this sound reasonable?
>
> It would sound reasonable if I had any tests to send to you :)
>
> Since I don't have any and since you're the one proposing the change, I
> guess it's up to you to write them. :)
Ah, but you owe a test debt from your earlier submission of kfifo! ;-)
Thanx, Paul
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