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

[Davidlohr Bueso]

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.

Thanks,
Davidlohr