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

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

On Mon, Nov 02, 2015 at 02:57:26PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 02, 2015 at 02:29:05PM +0100, Peter Zijlstra wrote:
> 
> > Note that while smp_cond_acquire() has an explicit
> > smp_read_barrier_depends() for Alpha, neither sites it gets used in
> > were actually buggy on Alpha for their lack of it. The first uses
> > smp_rmb(), which on Alpha is a full barrier too and therefore serves
> > its purpose. The second had an explicit full barrier.
> 
> > +/**
> > + * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
> > + * @cond: boolean expression to wait for
> > + *
> > + * Equivalent to using smp_load_acquire() on the condition variable but employs
> > + * the control dependency of the wait to reduce the barrier on many platforms.
> > + *
> > + * The control dependency provides a LOAD->STORE order, the additional RMB
> > + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
> > + * aka. ACQUIRE.
> > + */
> > +#define smp_cond_acquire(cond)	do {		\
> > +	while (!(cond))				\
> > +		cpu_relax();			\
> > +	smp_read_barrier_depends(); /* ctrl */	\
> > +	smp_rmb(); /* ctrl + rmb := acquire */	\
> > +} while (0)
> 
> So per the above argument we could leave out the
> smp_read_barrier_depends() for Alpha, although that would break
> consistency with all the other control dependency primitives we have. It
> would avoid issuing a double barrier.
> 
> Thoughts?

Do we even know that Alpha needs a barrier for control-dependencies in
the first place?

Will