Re: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

[Andrea Parri <parri.andrea@gmail.com>]

On Thu, Aug 30, 2018 at 05:31:32PM -0400, Alan Stern wrote:
> On Thu, 30 Aug 2018, Andrea Parri wrote:
> 
> > > 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>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Reviewed-by: Will Deacon <will.deacon@arm.com>
> > > Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > 
> > Round 2 ;-), I guess...  Let me start from the uncontroversial points:
> > 
> >   1) being able to use the LKMM to reason about generic locking code
> >      is useful and desirable (paraphrasing Peter in [1]);
> > 
> >   2) strengthening the ordering requirements of such code isn't going
> >      to boost performance (that's "real maths").
> > 
> > This patch is taking (1) away from us and it is formalizing (2), with
> > almost _no_ reason (no reason at all, if we stick to the commit msg.).
> 
> That's not quite fair.  Generic code isn't always universally
> applicable; some of it is opt-in -- meant only for the architectures
> that can support it.  In general, the LKMM allows us to reason about
> higher abstractions (such as locking) at a higher level, without
> necessarily being able to verify the architecture-specific details of
> the implementations.

No, generic code is "universally applicable" by definition; see below
for more on this point.


> 
> > In [2], Will wrote:
> > 
> >   "[...] having them [the RMWs] closer to RCsc[/to the semantics of
> >    locks] would make it easier to implement and reason about generic
> >    locking implementations (i.e. reduce the number of special ordering
> >    cases and/or magic barrier macros)"
> > 
> > "magic barrier macros" as in "mmh, if we accept this patch, we _should_
> > be auditing the various implementations/code to decide where to place a
> > 
> >   smp_barrier_promote_ordinary_release_acquire_to_unlock_lock()" ;-)
> > 
> > or the like, and "special ordering cases" as in "arrgh, (otherwise) we
> > are forced to reason on a per-arch basis while looking at generic code".
> 
> Currently the LKMM does not permit architecture-specific reasoning.  It 
> would have to be extended (in a different way for each architecture) 
> first.

Completely agreed; that's why I said that this patch is detrimental to
the applicability of the LKMM...


> 
> For example, one could use herd's POWER model combined with the POWER 
> compilation scheme and the POWER-specific implementation of spinlocks 
> for such reasoning.  The LKMM alone is not sufficient.
> 
> Sure, programming and reasoning about the kernel would be easier if all
> architectures were the same.  Unfortunately, we (and the kernel) have
> to live in the real world.
> 
> > (Remark: ordinary release/acquire are building blocks for code such as
> >  qspinlock, (q)rwlock, mutex, rwsem, ... and what else??).
> 
> But are these building blocks used the same way for all architectures?

The more, the better! (because then we have the LKMM tools) 

We already discussed the "fast path" example: the fast paths of the
above all resemble:

  *_lock(s):  atomic_cmpxchg_acquire(&s->val, UNLOCKED_VAL, LOCKED_VAL) ...
  *_unlock(s): ...  atomic_set_release(&s->val, UNLOCKED_VAL)

When I read this code, I think "Of course." (unless some arch. has
messed the implementation of cmpxchg_* up, which can happen...); but
then I read the subject line of this patch and I think "Wait, what?".

You can argue that this is not generic code, sure; but why on Earth
would you like to do so?!

  Andrea


> 
> > To avoid further repetition, I conclude by confirming all the concerns
> > and my assessment of this patch as pointed out in [3]; the subsequent
> > discussion, although not conclusive, presented several suggestions for
> > improvement (IMO).
> 
> Alan
>