Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

[Davidlohr Bueso]

On Fri, 17 Jun 2016, Davidlohr Bueso wrote:

> On Fri, 17 Jun 2016, Waiman Long wrote:
>
> > On 06/16/2016 09:11 PM, Davidlohr Bueso wrote:
> > > On Wed, 15 Jun 2016, Peter Zijlstra wrote:
> > >
> > > > Yeah, see a few patches further in this series, where he guards a
> > > > variables with the osq_lock.
> > >
> > > So one problem I have with all this is that if we are hardening 
> > > osq_lock/unlock()
> > > because of some future use that is specific to rwsems, then we 
> > > will immediately
> > > be hurting mutexes for no good reason.
> >
> > I am going to change it to use smp_acquire__after_ctrl_dep() as 
> > suggested by PeterZ. Is that a good enough compromise? I have also 
> > changed the xchg in the unlock side to xchg_release which could help 
> > performance in some archs. The thing is when developers see the name 
> > osq_lock/osq_unlock, they will naturally assume the proper barrriers 
> > are provided which is not currently the case.
>
> Oh, from your discussions with Boqun, I was under the impression that ->locked
> was now going to be properly ordered in all cases now, which is why
> I worry about mutexes.
>
> > Anyway, the change won't affect x86, it is probably ARM or PPC that 
> > may have an impact.
>
> Yes, that xchg() won't affect x86, but adding an smp_store_release(node->locked, 1)
> or such will obviously.

nm this last part, you're right, x86 smp_store_release is a nop.