Re: [PATCH v6 4/5] MCS Lock: Barrier corrections

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On Thu, Nov 21, 2013 at 01:56:16PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 21, 2013 at 12:03:08PM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 20, 2013 at 09:14:00AM -0800, Paul E. McKenney wrote:
> > > > Hmm, so in the following case:
> > > > 
> > > >   Access A
> > > >   unlock()	/* release semantics */
> > > >   lock()	/* acquire semantics */
> > > >   Access B
> > > > 
> > > > A cannot pass beyond the unlock() and B cannot pass the before the lock().
> > > > 
> > > > I agree that accesses between the unlock and the lock can be move across both
> > > > A and B, but that doesn't seem to matter by my reading of the above.
> > > > 
> > > > What is the problematic scenario you have in mind? Are you thinking of the
> > > > lock() moving before the unlock()? That's only permitted by RCpc afaiu,
> > > > which I don't think any architectures supported by Linux implement...
> > > > (ARMv8 acquire/release is RCsc).
> > > 
> > > If smp_load_acquire() and smp_store_release() are both implemented using
> > > lwsync on powerpc, and if Access A is a store and Access B is a load,
> > > then Access A and Access B can be reordered.
> > > 
> > > Of course, if every other architecture will be providing RCsc implementations
> > > for smp_load_acquire() and smp_store_release(), which would not be a bad
> > > thing, then another approach is for powerpc to use sync rather than lwsync
> > > for one or the other of smp_load_acquire() or smp_store_release().
> > 
> > So which of the two would make most sense?
> > 
> > As per the Document, loads/stores should not be able to pass up through
> > an ACQUIRE and loads/stores should not be able to pass down through a
> > RELEASE.
> > 
> > I think PPC would match that if we use sync for smp_store_release() such
> > that it will flush the store buffer, and thereby guarantee all stores
> > are kept within the required section.
> 
> Wouldn't that also mean that TSO archs need the full barrier on
> RELEASE?

It just might...  I was thinking not, but I do need to check.

							Thanx, Paul

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