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

From: Will Deacon
Date: Thu Jul 05 2018 - 12:21:53 EST

On Thu, Jul 05, 2018 at 08:44:39AM -0700, Daniel Lustig wrote:
> On 7/5/2018 8:31 AM, Paul E. McKenney wrote:
> > On Thu, Jul 05, 2018 at 10:21:36AM -0400, Alan Stern 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.
> >
> > Only in the presence of smp_mb__after_unlock_lock() or
> > smp_mb__after_spinlock(), correct? Or am I confused about RCsc?
> >
> > Thanx, Paul
> >
> In terms of what you're asking for really RCsc? To me,
> that would imply that even stores in the first critical section would
> need to be ordered before loads in the second critical section.
> Meaning that even x86 would need an mfence in either lock() or unlock()?

I think a LOCK operation always implies an atomic RmW, which will give
full ordering guarantees on x86. I know there have been interesting issues
involving I/O accesses in the past, but I think that's still out of scope
for the memory model.

Peter will know.