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

[Alan Stern]

On Tue, 10 Jul 2018, Paul E. McKenney wrote:

> 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@xxxxxxxxxxxxxxxxxxx>
> 
> It now applies, thank you very much!
> 
> Is this something that you are comfortable pushing into the upcoming
> merge window, or should I hold off until the next one?

Given the concerns that Andrea raised, and given that neither Peter, 
Will, nor Daniel has commented on v.3 of the patch, I think we should 
hold off for a little while.

Alan