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

[Alan Stern]

On Fri, 22 Jun 2018, Will Deacon wrote:

> Hi Alan,
> 
> 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,

What are the semantics of LDAPR?  That instruction isn't included in my
year-old copy of the ARMv8.1 manual; the closest it comes is LDAR and
LDAXP.

> so I'm surprised that RISC-V is ok with this. For example, the first test
> below is allowed on arm64.

Does ARMv8 use LDAPR for smp_load_aquire()?  If it doesn't, this is a 
moot point.

> I also think this would break if we used DMB LD to implement load-acquire
> (second test below).

Same question.

> So I'm not a big fan of this change, and I'm surprised this works on all
> architectures. What's the justification?

For ARMv8, I've been going by something you wrote in an earlier email
to the effect that store-release and load-acquire are fully ordered,
and therefore a release can never be forwarded to an acquire.  Is that
still true?  But evidently it only justifies patch 1 in this series,
not patch 2.

For RISC-V, I've been going by Andrea's and Luc's comments.

> > 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.
> 
> It's easier, but it will still break on architectures with native support
> for RCpc acquire/release.

Again, do we want the kernel to support that?

For that matter, what would happen if someone were to try using RCpc 
semantics for lock/unlock?  Or to put it another way, why do you 
contemplate the possibility of RCpc acquire/release but not RCpc 
lock/unlock?

> Could we drop the acquire/release stuff from the patch and limit this change
> to locking instead?

The LKMM uses the same CAT code for acquire/release and lock/unlock.  
(In essence, it considers a lock to be an acquire and an unlock to be a
release; everything else follows from that.)  Treating one differently
from the other in these tests would require some significant changes.
It wouldn't be easy.

Alan