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

[Paul E. McKenney]

On Thu, Jan 17, 2019 at 02:43:54PM -0500, Alan Stern wrote:
> On Wed, 16 Jan 2019, Andrea Parri wrote:
> 
> > Can the compiler (maybe, it does?) transform, at the C or at the "asm"
> > level, LB1's P0 in LB2's P0 (LB1 and LB2 are reported below)?
> > 
> > C LB1
> > 
> > {
> > 	int *x = &a;
> > }
> > 
> > P0(int **x, int *y)
> > {
> > 	int *r0;
> > 
> > 	r0 = rcu_dereference(*x);
> > 	*r0 = 0;
> > 	smp_wmb();
> > 	WRITE_ONCE(*y, 1);
> > }
> > 
> > P1(int **x, int *y, int *b)
> > {
> > 	int r0;
> > 
> > 	r0 = READ_ONCE(*y);
> > 	rcu_assign_pointer(*x, b);
> > }
> > 
> > exists (0:r0=b /\ 1:r0=1)
> > 
> > 
> > C LB2
> > 
> > {
> > 	int *x = &a;
> > }
> > 
> > P0(int **x, int *y)
> > {
> > 	int *r0;
> > 
> > 	r0 = rcu_dereference(*x);
> > 	if (*r0)
> > 		*r0 = 0;
> > 	smp_wmb();
> > 	WRITE_ONCE(*y, 1);
> > }
> > 
> > P1(int **x, int *y, int *b)
> > {
> > 	int r0;
> > 
> > 	r0 = READ_ONCE(*y);
> > 	rcu_assign_pointer(*x, b);
> > }
> > 
> > exists (0:r0=b /\ 1:r0=1)
> > 
> > LB1 and LB2 are data-race free, according to the patch; LB1's "exists"
> > clause is not satisfiable, while LB2's "exists" clause is satisfiable.
> 
> Umm.  Transforming
> 
> 	*r0 = 0;
> 
> to
> 
> 	if (*r0 != 0)
> 		*r0 = 0;
> 
> wouldn't work on Alpha if r0 was assigned from a plain read with no
> memory barrier between.  But when r0 is assigned from an
> rcu_dereference call, or if there's no indirection (as in "if (a != 0)
> a = 0;"), the compiler is indeed allowed to perform this
> transformation.
> 
> This means my definition of preserved writes was wrong; a write we 
> thought had to be preserved could instead be transformed into a read.
> 
> This objection throws a serious monkey wrench into my approach.  For
> one thing, it implies that (as in the example) we can't expect
> smp_wmb() always to order plain writes.  For another, it means we have
> to assume a lot more writes need not be preserved.
> 
> I don't know.  This may doom the effort to formalize dependencies to
> plain accesses.  Or at least, those other than address dependencies
> from marked reads.

(Catching up, hello from Auckland!)

At this point, I am very much in favor of taking the simpler starting
point.  If someone is using any sort of dependency from a plain access,
all bets are off.  Similarly, if someone is using a control or data
dependency even from a marked access, the later dependent access must
be marked to guarantee ordering.

I believe that the transformation from "*r0 = 0" should be convincing.  ;-)

							Thanx, Paul