Re: [RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to work with multiple kinds of dependencies

Peter Zijlstra
2018-02-22T15:30:43Z

On Thu, Feb 22, 2018 at 11:12:10PM +0800, Boqun Feng wrote:
> On Thu, Feb 22, 2018 at 03:26:14PM +0100, Peter Zijlstra wrote:

> > However, I would suggest:
> >
> > static inline bool is_xr(u16 dep)
> > {
> > return !!(dep & (DEP_NR_MASK | DEP_RR_MASK));
> > }
> >
> > static inline bool is_rx(u16 dep)
> > {
> > return !!(dep & (DEP_RN_MASK | DEP_RR_MASK));
> > }
> >
> >
> > > @@ -1095,11 +1179,18 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
> > > else
> > > head = &lock->class->locks_before;
> > >
> > > + is_rr = lock->is_rr;
> > > +
> > > DEBUG_LOCKS_WARN_ON(!irqs_disabled());
> > >
> > > list_for_each_entry_rcu(entry, head, entry) {
> > > unsigned int cq_depth;
> > >
> > > + next_is_rr = pick_dep(is_rr, entry->dep);
> > > + if (next_is_rr < 0)
> > > + continue;
> > > + entry->is_rr = next_is_rr;
> >
> > /* Skip *R -> R* relations */
> > if (have_xr && is_rx(entry->dep))
> > continue;
>
> I don't think this works, if we pick a *R for previous entry, and for
> current entry, we have RR, NN and NR, your approach will skip the
> current entry, but actually we can pick NN or NR (of course, in __bfs(),
> we can greedily pick NN, because if NR causes a deadlock, so does NN).

I don't get it, afaict my suggestion is identical.

You skip condition: pick_dep() < 0, evaluates to:

is_rr && (!NN_MASK && !NR_MASK) :=
is_rr && (RN_MASK | RR_MASK)

Which is exactly what I have.

If that is satisfied, you set entry->is_rr to pick_dep(), which his
harder to evaluate, but is something like:

is_rr && NR_MASK || !(NN_MASK | RN_MASK) :=
is_rr && NR_MASK || (NR_MASK | RR_MASK) :=
(NR_MASK | RR_MASK)

(because is_rr && RR_MASK will have been skipped)

> >
> > entry->have_xr = is_xr(entry->dep);
> >
> > Which to me is a much simpler construct, hmm?