Re: [PATCH v6 4/5] MCS Lock: Barrier corrections

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 26, 2013 at 11:00:50AM -0800, Linus Torvalds wrote:
> On Tue, Nov 26, 2013 at 1:59 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > If you now want to weaken this definition, then that needs consideration
> > because we actually rely on things like
> >
> > spin_unlock(l1);
> > spin_lock(l2);
> >
> > being full barriers.
> 
> Btw, maybe we should just stop that assumption.

I'd be fine with that; it was one of the options listed. I was just
somewhat concerned that the definitions given by the Document and the
reality of proposed implementations was drifting.

> IOW, where do we really care about the "unlock+lock" is a memory
> barrier? And could we make those places explicit, and then do
> something similar to the above to them?

So I don't know :-(

I do know myself and Oleg have often talked about it, and I'm fairly
sure we must have used it at some point.

I think that introduction of smp_mb__before_spinlock() actually killed a
few of those, but I can't recall.

Oleg doesn't actually seem to be on the CC list -- lets amend that.

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