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

[Will Deacon]

On Wed, Nov 29, 2017 at 08:46:02PM +0100, 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.

I don't think it's actually that weird. If, for example, the write to *y in
P0 was part of an UNLOCK operation and the load_acquire of y in P1 was a
LOCK operation, then the xchg could just be setting some waiting bit in
other bits of the lock word. C/C++ also requires order here if the xchg is
done on its own thread.

Will