Re: Plain accesses and data races in the Linux Kernel Memory Model

[Peter Zijlstra]

On Tue, Jan 15, 2019 at 10:19:10AM -0500, Alan Stern wrote:
> On Tue, 15 Jan 2019, Andrea Parri wrote:
> 
> > Unless I'm mis-reading/-applying this definition, this will flag the
> > following test (a variation on your "race.litmus") with "data-race":
> > 
> > C no-race
> > 
> > {}
> > 
> > P0(int *x, spinlock_t *s)
> > {
> > 	spin_lock(s);
> >         WRITE_ONCE(*x, 1);	/* A */
> > 	spin_unlock(s);	/* B */
> > }
> > 
> > P1(int *x, spinlock_t *s)
> > {
> >         int r1;
> > 
> > 	spin_lock(s); /* C */
> >         r1 = *x;	/* D */
> > 	spin_unlock(s);
> > }
> > 
> > exists (1:r1=1)
> > 
> > Broadly speaking, this is due to the fact that the modified "happens-
> > before" axiom does not forbid the execution with the (MP-) cycle
> > 
> > 	A ->po-rel B ->rfe C ->acq-po D ->fre A
> > 
> > and then to the link "D ->race-from-r A" here defined.
> 
> Yes, that cycle certainly should be forbidden.  On the other hand, we
> don't want to insist that C happens before D, given that D may not
> happen at all.
> 
> This is a real problem.  Can we solve it by adding a modified
> "happens-before" which says essentially that _if_ D is preserved _then_
> C happens before D?  But then what about cycles involving more than one
> possibly preserved access?  Or maybe a relation which says that D
> cannot execute before C (so if D executes at all, it has to come after
> C)?

The latter; there is a compiler barrier implied at the end of
spin_lock() such that anything later (in PO) must indeed be later.

> Now you see why this stuff is so difficult...  At the moment, I don't
> know how to fix this.