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

Paul E. McKenney
2018-07-11T18:01:12Z

On Wed, Jul 11, 2018 at 06:17:17PM +0200, Andrea Parri wrote:
> On Wed, Jul 11, 2018 at 08:42:11AM -0700, Paul E. McKenney 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>
> >
> > 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.
>
> I raised some concerns in my review to v2; AFAICT, these concerns have
> not been resolved: so, until then, please feel free to add my NAK. ;-)

I will be keeping the prototype in my -rcu tree for the time being,
but I will not be submitting it into the v4.19 merge window. I expect
that you all will be able to come to agreement in the couple of months
until the v4.20/v5.0 merge window. ;-)

I will apply Peter's ack and at the same time mark it EXP so that its
state will be apparent.

Thanx, Paul