Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

From: Paul E. McKenney
Date: Fri Jul 06 2018 - 17:08:49 EST

On Fri, Jul 06, 2018 at 04:37:21PM -0400, Alan Stern wrote:
> On Thu, 5 Jul 2018, Andrea Parri wrote:
> > > At any rate, it looks like instead of strengthening the relation, I
> > > should write a patch that removes it entirely. I also will add new,
> > > stronger relations for use with locking, essentially making spin_lock
> > > and spin_unlock be RCsc.
> >
> > Thank you.
> >
> > Ah let me put this forward: please keep an eye on the (generic)
> >
> > queued_spin_lock()
> > queued_spin_unlock()
> >
> > (just to point out an example). Their implementation (in part.,
> > the fast-path) suggests that if we will stick to RCsc lock then
> > we should also stick to RCsc acq. load from RMW and rel. store.
> A very good point. The implementation of those routines uses
> atomic_cmpxchg_acquire() to acquire the lock. Unless this is
> implemented with an operation or fence that provides write-write
> ordering (in conjunction with a suitable release), qspinlocks won't
> have the ordering properties that we want.
> I'm going to assume that the release operations used for unlocking
> don't need to have any extra properties; only the lock-acquire
> operations need to be special (i.e., stronger than a normal
> smp_load_acquire). This suggests that atomic RMW functions with acquire
> semantics should also use this stronger form of acquire.
> Does anybody have a different suggestion?

The approach you suggest makes sense to me. Will, Peter, Daniel, any
reasons why this approach would be a problem for you guys?

Thanx, Paul