Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Andrea Parri
2018-07-11T12:34:34Z

On Wed, Jul 11, 2018 at 10:43:11AM +0100, Will Deacon wrote:
> On Tue, Jul 10, 2018 at 11:38:21AM +0200, Andrea Parri wrote:
> > On Mon, Jul 09, 2018 at 04:01:57PM -0400, Alan Stern wrote:
> > > More than one kernel developer has expressed the opinion that the LKMM
> > > should enforce ordering of writes by locking. In other words, given
> >
> > I'd like to step back on this point: I still don't have a strong opinion
> > on this, but all this debating made me curious about others' opinion ;-)
> > I'd like to see the above argument expanded: what's the rationale behind
> > that opinion? can we maybe add references to actual code relying on that
> > ordering? other that I've been missing?
> >
> > I'd extend these same questions to the "ordering of reads" snippet below
> > (and discussed since so long...).
> >
> >
> > > the following code:
> > >
> > > WRITE_ONCE(x, 1);
> > > spin_unlock(&s):
> > > spin_lock(&s);
> > > 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. In terms of
> > > the memory model, this means expanding the cumul-fence relation.
> > >
> > > Locks should also provide read-read (and read-write) ordering in a
> > > similar way. Given:
> > >
> > > READ_ONCE(x);
> > > spin_unlock(&s);
> > > spin_lock(&s);
> > > READ_ONCE(y); // or WRITE_ONCE(y, 1);
> > >
> > > the load of x should be executed before the load of (or store to) y.
> > > The LKMM already provides this ordering, but it provides it even in
> > > the case where the two accesses are separated by a release/acquire
> > > pair of fences rather than unlock/lock. This would prevent
> > > architectures from using weakly ordered implementations of release and
> > > acquire, which seems like an unnecessary restriction. The patch
> > > therefore removes the ordering requirement from the LKMM for that
> > > case.
> >
> > IIUC, the same argument could be used to support the removal of the new
> > unlock-rf-lock-po (we already discussed riscv .aq/.rl, it doesn't seem
> > hard to imagine an arm64 LDAPR-exclusive, or the adoption of ctrl+isync
> > on powerpc). Why are we effectively preventing their adoption? Again,
> > I'd like to see more details about the underlying motivations...
> >
> >
> > >
> > > All the architectures supported by the Linux kernel (including RISC-V)
> > > do provide this ordering for locks, albeit for varying reasons.
> > > Therefore this patch changes the model in accordance with the
> > > developers' wishes.
> > >
> > > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> > >
> > > ---
> > >
> > > v.2: Restrict the ordering to lock operations, not general release
> > > and acquire fences.
> >
> > This is another controversial point, and one that makes me shivering ...
> >
> > I have the impression that we're dismissing the suggestion "RMW-acquire
> > at par with LKR" with a bit of rush. So, this patch is implying that:
> >
> > while (cmpxchg_acquire(&s, 0, 1) != 0)
> > cpu_relax();
> >
> > is _not_ a valid implementation of spin_lock()! or, at least, it is not
> > when paired with an smp_store_release(). Will was anticipating inserting
> > arch hooks into the (generic) qspinlock code, when we know that similar
> > patterns are spread all over in (q)rwlocks, mutexes, rwsem, ... (please
> > also notice that the informal documentation is currently treating these
> > synchronization mechanisms equally as far as "ordering" is concerned...).
> >
> > This distinction between locking operations and "other acquires" appears
> > to me not only unmotivated but also extremely _fragile (difficult to use
> > /maintain) when considering the analysis of synchronization mechanisms
> > such as those mentioned above or their porting for new arch.
>
> The main reason for this is because developers use spinlocks all of the
> time, including in drivers. It's less common to use explicit atomics and
> extremely rare to use explicit acquire/release operations. So let's make
> locks as easy to use as possible, by giving them the strongest semantics
> that we can whilst remaining a good fit for the instructions that are
> provided by the architectures we support.

Simplicity is the eye of the beholder. From my POV (LKMM maintainer), the
simplest solution would be to get rid of rfi-rel-acq and unlock-rf-lock-po
(or its analogous in v3) all together:

diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index 59b5cbe6b6240..bc413a6839a2d 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -38,7 +38,6 @@ let strong-fence = mb | gp
(* Release Acquire *)
let acq-po = [Acquire] ; po ; [M]
let po-rel = [M] ; po ; [Release]
-let rfi-rel-acq = [Release] ; rfi ; [Acquire]

(**********************************)
(* Fundamental coherence ordering *)
@@ -60,7 +59,7 @@ let dep = addr | data
let rwdep = (dep | ctrl) ; [W]
let overwrite = co | fr
let to-w = rwdep | (overwrite & int)
-let to-r = addr | (dep ; rfi) | rfi-rel-acq
+let to-r = addr | (dep ; rfi)
let fence = strong-fence | wmb | po-rel | rmb | acq-po
let ppo = to-r | to-w | fence

Among other things, this would immediately:

1) Enable RISC-V to use their .aq/.rl annotations _without_ having to
"worry" about tso or release/acquire fences; IOW, this will permit
a partial revert of:

0123f4d76ca6 ("riscv/spinlock: Strengthen implementations with fences")
5ce6c1f3535f ("riscv/atomic: Strengthen implementations with fences")

2) Resolve the above mentioned controversy (the inconsistency between
- locking operations and atomic RMWs on one side, and their actual
implementation in generic code on the other), thus enabling the use
of LKMM _and_ its tools for the analysis/reviewing of the latter.

>
> If you want to extend this to atomic rmws, go for it, but I don't think
> it's nearly as important and there will still be ways to implement locks
> with insufficient ordering guarantees if you want to.

I don't want to "implement locks with insufficient ordering guarantees"
(w.r.t. LKMM). ;-)

Andrea

>
> Will