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

[Andrea Parri]

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@xxxxxxxxxxxxxxxxxxx>
> > 
> > Thanks, I'm happy with this version of the patch:
> > 
> > Reviewed-by: Will Deacon <will.deacon@xxxxxxx>
> 
> 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. ;-)

  Andrea


> 
> 							Thanx, Paul
>