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.

Reply via email to