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

[Will Deacon <will.deacon@arm.com>]

On Mon, Nov 02, 2015 at 11:17:17AM -0800, Linus Torvalds wrote:
> On Mon, Nov 2, 2015 at 10:37 AM, Will Deacon <will.deacon@arm.com> wrote:
> > On Mon, Nov 02, 2015 at 10:08:24AM -0800, Linus Torvalds wrote:
> >> On Mon, Nov 2, 2015 at 5:29 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >> > +#define smp_cond_acquire(cond) do {            \
> >> > +       while (!(cond))                         \
> >> > +               cpu_relax();                    \
> >> > +       smp_read_barrier_depends(); /* ctrl */  \
> >> > +       smp_rmb(); /* ctrl + rmb := acquire */  \
> >> > +} while (0)
> >>
> >> This code makes absolutely no sense.
> >>
> >> 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.

> As to READ_ONCE_CTRL - two wrongs don't make a right.

Sure, I just wanted to point out the precedence and related discussion.

> 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?

> > In this case, control dependencies are only referring to READ -> WRITE
> > ordering, so they are honoured by ARM and PowerPC.
> 
> Do ARM and PPC actually guarantee the generic "previous reads always
> order before subsequent writes"?

Only to *dependent* subsequent writes, but Peter's patch makes all
subsequent writes dependent on cond, so I think that's the right way to
achieve the ordering we want.

Will