Re: [PATCH v3] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Paul E. McKenney
2018-07-11T18:08:31Z

On Wed, Jul 11, 2018 at 06:34:56PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 11, 2018 at 10:43:45AM +0100, Will Deacon wrote:
> > Hi Alan,
> >
> > On Tue, Jul 10, 2018 at 02:18:13PM -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>
> >
> > Thanks, I'm happy with this version of the patch:
> >
> > Reviewed-by: Will Deacon <will.deacon@arm.com>
>
> Me too! Thanks Alan.
>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

And I applies you ask as well, thank you!

Thanx, Paul