Re: [RFC PATCH-tip v4 01/10] locking/osq: Make lock/unlock proper acquire/release barrier

[Waiman Long]

On 10/06/2016 01:47 AM, Davidlohr Bueso wrote:
> On Wed, 05 Oct 2016, Waiman Long wrote:
>
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index 05a3785..1e6823a 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -12,6 +12,23 @@
> >  */
> > static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, 
> > osq_node);
> >
> > +enum mbtype {
> > +    acquire,
> > +    release,
> > +    relaxed,
> > +};
>
> No, please.
>
> > +
> > +static __always_inline int
> > +_atomic_cmpxchg_(const enum mbtype barrier, atomic_t *v, int old, 
> > int new)
> > +{
> > +    if (barrier == acquire)
> > +        return atomic_cmpxchg_acquire(v, old, new);
> > +    else if (barrier == release)
> > +        return atomic_cmpxchg_release(v, old, new);
> > +    else
> > +        return atomic_cmpxchg_relaxed(v, old, new);
> > +}
>
> Things like the above are icky. How about something like below? I'm not
> crazy about it, but there are other similar macros, ie lockref. We still
> provide the osq_lock/unlock to imply acquire/release and the new _relaxed
> flavor, as I agree that should be the correct naming
>
> While I have not touched osq_wait_next(), the following are impacted:
>
> - node->locked is now completely without ordering for _relaxed() 
> (currently
> its under smp_load_acquire, which does not match and the race is harmless
> to begin with as we just iterate again. For the acquire flavor, it is 
> always
> formed with ctr dep + smp_rmb().
>
> - If osq_lock() fails we never guarantee any ordering.
>
> What do you think?
>
> Thanks,
> Davidlohr

Yes, I am OK with your change. However, I need some additional changes 
in osq_wait_next() as well. Either it is changed to use the release 
variants of atomic_cmpxchg and xchg or using macro like what you did 
with osq_lock and osq_unlock. The release variant is needed in the 
osq_lock(). As osq_wait_next() is only invoked in the failure path of 
osq_lock(), the barrier type doesn't really matter.

Cheers,
Longman
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