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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.

Yep, for PPC we can just use sync for smp_store_release().  We just need
to check the other architectures.

							Thanx, Paul

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