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

Andrea Parri
2018-07-13T19:06:53Z

On Fri, Jul 13, 2018 at 10:16:48AM -0700, Linus Torvalds wrote:
> On Fri, Jul 13, 2018 at 2:34 AM Will Deacon <will.deacon@arm.com> wrote:
> >
> > And, since we're stating preferences, I'll reiterate my preference towards:
> >
> > * RCsc unlock/lock
> > * RCpc release/acquire
>
> Yes, I think this would be best. We *used* to have pretty heavy-weight
> locking rules for various reasons, and we relaxed them for reasons
> that weren't perhaps always the right ones.
>
> Locking is pretty heavy-weight in general, and meant to be the "I
> don't really have to think about this very much" option. Then not
> being serializing enough to confuse people when it allows odd behavior
> (on _some_ architectures) does not sound like a great idea.
>
> In contrast, when you do release/acquire or any of the other "I know
> what I'm doing" things, I think we want the minimal serialization
> implied by the very specialized op.

The changes under discussion are _not_ affecting uses such as:

P0:
spin_lock(s);
UPDATE data_struct
spin_unlock(s);

P1:
spin_lock(s);
UPDATE data_struct
spin_unlock(s);

[...]

(most common use case for locking?): these uses work just _fine_ with
the current implementations and in LKMM.

OTOH, these changes are going to affect uses where threads interact by
"mixing" locking and _other_ synchronization primitives such as in:

{ x = 0; y = 0; }

P0:
spin_lock(s);
WRITE_ONCE(x, 1);
spin_unlock(s);

P1:
spin_lock(s);
r0 = READ_ONCE(x);
WRITE_ONCE(y, 1);
spin_unlock(s);

P2:
r1 = smp_load_acquire(&y);
r2 = READ_ONCE(x);

BUG_ON(r0 == 1 && r1 == 1 && r2 == 0)

and

{ x = 0; y = 0; z = 0; }

P0:
spin_lock(s);
WRITE_ONCE(x, 1);
r0 = READ_ONCE(y);
spin_unlock(s);

P1:
spin_lock(s);
WRITE_ONCE(y, 1);
r1 = READ_ONCE(z);
spin_unlock(s);

P2
WRITE_ONCE(z, 1);
smp_mb();
r2 = READ_ONCE(x);

BUG_ON(r0 == 0 && r1 == 0 && r2 == 0)

(inspired from __two__ uses in kernel/{sched,rcu}). Even if someone were
to tell me that locks serialize enough, I'd still be prompted to say "yes,
but do / can my BUG_ON()s fire?".

Actually, my very first reaction, before starting what does appear to be
indeed a long and complex conversation, would probably be to run/check the
above snippets against the (latest) LKMM, by using the associated tool.

Once "checked" with both people and automated models, I'd probably remain
suspicious about my "magic" code so that I most likely will be prompted to
dig into each single arch. implementation / reference manual...

... Time's up!

Andrea