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

[Alan Stern]

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.

Alan