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

[Peter Zijlstra]

On Fri, Jun 22, 2018 at 10:55:47AM +0100, Will Deacon wrote:
> 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);

So this is the one I'm relying on and really want sorted.

> > > or the following:
> > > 
> > > 	smp_store_release(&x, 1);
> > > 	r1 = smp_load_acquire(&x);	// r1 = 1
> > > 	WRITE_ONCE(y, 1);

Reading back some of the old threads [1], it seems the direct
translation of the first into acquire-release would be:

	WRITE_ONCE(x, 1);
	smp_store_release(&s, 1);
	r1 = smp_load_acquire(&s);
	WRITE_ONCE(y, 1);

Which is I think easier to make happen than the second example you give.

> > > 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.

They in fact do so on chips lacking LWSYNC, see how PPC_ACQUIRE_BARRIER
(as used by atomic_*_acquire) turns into ISYNC (note however that they
do not use PPC_ACQUIRE_BARRIER for smp_load_acquire -- because there's
no CTRL there).


[1] https://lkml.kernel.org/r/20171128095850.rhtnx6e2qxep5npa@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx