Re: Do we need to correct barriering in circular-buffers.rst?

Peter Zijlstra
2019-09-27T15:57:46Z

On Fri, Sep 27, 2019 at 02:49:29PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 27, 2019 at 11:51:07AM +0200, Andrea Parri wrote:
>
> > For the record, the LKMM doesn't currently model "order" derived from
> > control dependencies to a _plain_ access (even if the plain access is
> > a write): in particular, the following is racy (as far as the current
> > LKMM is concerned):
> >
> > C rb
> >
> > { }
> >
> > P0(int *tail, int *data, int *head)
> > {
> > if (READ_ONCE(*tail)) {
> > *data = 1;
> > smp_wmb();
> > WRITE_ONCE(*head, 1);
> > }
> > }
> >
> > P1(int *tail, int *data, int *head)
> > {
> > int r0;
> > int r1;
> >
> > r0 = READ_ONCE(*head);
> > smp_rmb();
> > r1 = *data;
> > smp_mb();
> > WRITE_ONCE(*tail, 1);
> > }
> >
> > Replacing the plain "*data = 1" with "WRITE_ONCE(*data, 1)" (or doing
> > s/READ_ONCE(*tail)/smp_load_acquire(tail)) suffices to avoid the race.
> > Maybe I'm short of imagination this morning... but I can't currently
> > see how the compiler could "break" the above scenario.
>
> The compiler; if sufficiently smart; is 'allowed' to change P0 into
> something terrible like:
>
> *data = 1;
> if (*tail) {
> smp_wmb();
> *head = 1;
> } else
> *data = 0;
>
>
> (assuming it knows *data was 0 from a prior store or something)
>
> Using WRITE_ONCE() defeats this because volatile indicates external
> visibility.

The much simpler solution might be writing it like:

if (READ_ONCE(*tail) {
barrier();
*data = 1;
smp_wmb();
WRITE_ONCE(*head, 1);
}

which I don't think the compiler is allowed to mess up.