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

Alan Stern
2018-07-10T13:57:23Z

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.

Alan