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

Paul E. McKenney
2018-07-10T16:23:49Z

On Tue, Jul 10, 2018 at 09:57:17AM -0400, Alan Stern wrote:
> On Mon, 9 Jul 2018, Paul E. McKenney 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
> > > 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.
> > >
> > > 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>
> >
> > Nice!
> >
> > However, it doesn't apply against current -rcu. Am I missing a patch?
> > Or is this supposed to apply against origin/lkmm?
>
> I wrote it based on 4.18-rc. However, I can rebase it against your
> current dev branch.

Could you please? Against either the dev or lkmm branch should well.

If you don't have time for this, my approach would be to apply against
4.18-rc, then cherry-pick onto my branch, resolving the conflicts and
emailing you both the "<<<<"-marked file and my proposed resolution.
(Or git might just resolve everything automatically -- that does
sometimes happen. But it would still be good to double-check its work,
as it sometimes does "interesting" resolutions.)

Thanx, Paul