Re: Unlock-lock questions and the Linux Kernel Memory Model

[Paul E. McKenney]

On Wed, Nov 29, 2017 at 02:53:06PM -0500, Alan Stern wrote:
> On Wed, 29 Nov 2017, Peter Zijlstra wrote:
> 
> > On Wed, Nov 29, 2017 at 11:04:53AM -0800, Daniel Lustig wrote:
> > 
> > > While we're here, let me ask about another test which isn't directly
> > > about unlock/lock but which is still somewhat related to this
> > > discussion:
> > > 
> > > "MP+wmb+xchg-acq" (or some such)
> > > 
> > > {}
> > > 
> > > P0(int *x, int *y)
> > > {
> > >         WRITE_ONCE(*x, 1);
> > >         smp_wmb();
> > >         WRITE_ONCE(*y, 1);
> > > }
> > > 
> > > P1(int *x, int *y)
> > > {
> > >         r1 = atomic_xchg_relaxed(y, 2);
> > >         r2 = smp_load_acquire(y);
> > >         r3 = READ_ONCE(*x);
> > > }
> > > 
> > > exists (1:r1=1 /\ 1:r2=2 /\ 1:r3=0)
> > > 
> > > C/C++ would call the atomic_xchg_relaxed part of a release sequence
> > > and hence would forbid this outcome.
> > 
> > That's just weird. Either its _relaxed, or its _release. Making _relaxed
> > mean _release is just daft.
> 
> The C11 memory model specifically allows atomic operations to be 
> interspersed within a release sequence.  But it doesn't say why.

The use case put forward within the committee is for atomic quantities
with mode bits.  The most frequent has the atomic quantity having
lock-like properties, in which case you don't want to lose the ordering
effects of the lock handoff just because a mode bit got set or cleared.
Some claim to actually use something like this, but details have not
been forthcoming.

I confess to being a bit skeptical.  If the mode changes are infrequent,
the update could just as well be ordered.

That said, Daniel, the C++ memory model really does require that the
above litmus test be forbidden, my denigration of it notwithstanding.

							Thanx, Paul