Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 02, 2015 at 07:57:09PM +0000, Will Deacon wrote:
> > >> smp_read_barrier_depends() is about a memory barrier where there is a
> > >> data dependency between two accesses. The "depends" is very much about
> > >> the data dependency, and very much about *nothing* else.
> > >
> > > Paul wasn't so sure, which I think is why smp_read_barrier_depends()
> > > is already used in, for example, READ_ONCE_CTRL:
> > >
> > >   http://lkml.kernel.org/r/20151007154003.GJ3910@linux.vnet.ibm.com
> > 
> > Quoting the alpha architecture manual is kind of pointless, when NO
> > OTHER ARCHITECTURE OUT THERE guararantees that whole "read +
> > conditional orders wrt subsequent writes" _either_.
> > 
> > (Again, with the exception of x86, which has the sane "we honor causality")
> > 
> > Alpha isn't special. And smp_read_barrier_depends() hasn't magically
> > become something new.
> > 
> > If people think that control dependency needs a memory barrier on
> > alpha, then it damn well needs on on all other weakly ordered
> > architectuers too, afaik.
> > 
> > Either that "you cannot finalize a write unless all conditionals it
> > depends on are finalized" is true or it is not. That argument has
> > *never* been about some architecture-specific memory ordering model,
> > as far as I know.
> 
> You can imagine a (horribly broken) value-speculating architecture that
> would permit this re-ordering, but then you'd have speculative stores
> and thin-air values, which Alpha doesn't have.

I've tried arguing this point with Paul on a number of occasions, and I
think he at one point constructed some elaborate scheme that depended on
the split cache where this might require the full barrier.

> > As to READ_ONCE_CTRL - two wrongs don't make a right.
> 
> Sure, I just wanted to point out the precedence and related discussion.

I purely followed 'convention' here.

> > That smp_read_barrier_depends() there doesn't make any sense either.
> > 
> > And finally, the alpha architecture manual actually does have the
> > notion of "Dependence Constraint" (5.6.1.7) that talks about writes
> > that depend on previous reads (where "depends" is explicitly spelled
> > out to be about conditionals, write data _or_ write address). They are
> > actually constrained on alpha too.
> 
> In which case, it looks like we can remove the smp_read_barrier_depends
> instances from all of the control-dependency macros, but I'll defer to
> Paul in case he has some further insight. I assume you're ok with this
> patch if the smp_read_barrier_depends() is removed?

That would be good indeed.