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

[Will Deacon]

On Fri, Jun 22, 2018 at 09:09:28AM +0100, Will Deacon wrote:
> On Thu, Jun 21, 2018 at 01:27:12PM -0400, Alan Stern wrote:
> > More than one kernel developer has expressed the opinion that the LKMM
> > should enforce ordering of writes by release-acquire chains and by
> > locking.  In other words, given the following code:
> > 
> > 	WRITE_ONCE(x, 1);
> > 	spin_unlock(&s):
> > 	spin_lock(&s);
> > 	WRITE_ONCE(y, 1);
> > 
> > or the following:
> > 
> > 	smp_store_release(&x, 1);
> > 	r1 = smp_load_acquire(&x);	// r1 = 1
> > 	WRITE_ONCE(y, 1);
> > 
> > the stores to x and y should be propagated in order to all other CPUs,
> > even though those other CPUs might not access the lock s or be part of
> > the release-acquire chain.  In terms of the memory model, this means
> > that rel-rf-acq-po should be part of the cumul-fence relation.
> > 
> > All the architectures supported by the Linux kernel (including RISC-V)
> > do behave this way, albeit for varying reasons.  Therefore this patch
> > changes the model in accordance with the developers' wishes.
> 
> Interesting...
> 
> I think the second example would preclude us using LDAPR for load-acquire,
> so I'm surprised that RISC-V is ok with this. For example, the first test
> below is allowed on arm64.
> 
> I also think this would break if we used DMB LD to implement load-acquire
> (second test below).
> 
> So I'm not a big fan of this change, and I'm surprised this works on all
> architectures. What's the justification?

I also just realised that this prevents Power from using ctrl+isync to
implement acquire, should they wish to do so.

Will