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

Paul E. McKenney
2018-07-11T15:40:13Z

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>

I have applied your Reviewed-by, and thank you both!

Given that this is a non-trivial change and given that I am posting
for -tip acceptance in a few days, I intend to send this one not
to the upcoming merge window, but to the one after that.

Please let me know if there is an urgent need for this to go into the
v4.19 merge window.

Thanx, Paul