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

Peter Zijlstra
2018-02-22T14:26:23Z

On Thu, Feb 22, 2018 at 03:08:52PM +0800, Boqun Feng wrote:
> Now we have four kinds of dependencies in the dependency graph, and not
> all the pathes carry strong dependencies, for example:
>
> Given lock A, B, C, if we have:
>
> CPU1 CPU2
> ============= ==============
> write_lock(A); read_lock(B);
> read_lock(B); write_lock(C);
>
> then we have dependencies A--(NR)-->B, and B--(RN)-->C, (NR and
> RN are to indicate the dependency kind), A actually doesn't have
> strong dependency to C(IOW, C doesn't depend on A), to see this,
^ missing space

You're fairly consistent with not putting spaces before opening braces
in text, please don't do this, this is not a C function call. Also
double check all your braces are matched, IIRC there's at least one that
isn't closed in the previous patches.

> let's say we have a third CPU3 doing:
>
> CPU3:
> =============
> write_lock(C);
> write_lock(A);
>
> , this is not a deadlock. However if we change the read_lock()
> on CPU2 to a write_lock(), it's a deadlock then.
>
> So A --(NR)--> B --(RN)--> C is not a strong dependency path but
> A --(NR)--> B --(NN)-->C is a strong dependency path.

I'm not really satisfied with the above reasoning. I don't disagree, but
if possible it would be nice to have something a little more solid.

> We can generalize this as: If a path of dependencies doesn't have two
> adjacent dependencies as (*R)--L-->(R*), where L is some lock, it is a
> strong dependency path, otherwise it's not.
>
> Now our mission is to make __bfs() traverse only the strong dependency
> paths, which is simple: we record whether we have -(*R)-> at the current
> tail of the path in lock_list::is_rr, and whenever we pick a dependency
> in the traverse, we 1) make sure we don't pick a -(R*)-> dependency if
> our current tail is -(*R)-> and 2) greedily pick a -(*N)-> as hard as
> possible.
>
> With this extension for __bfs(), we now need to initialize the root of
> __bfs() properly(with a correct ->is_rr), to do so, we introduce some
^ another missing space

Also, I don't like is_rr as a name, have_xr might be better.

Only if we combine *R with R* do we have RR.

> +/*
> + * return -1 if no proper dependency could be picked
> + * return 0 if a -(*N)-> dependency could be picked
> + * return 1 if only a -(*R)-> dependency could be picked
> + *
> + * N: non-recursive lock
> + * R: recursive read lock
> + */
> +static inline int pick_dep(u16 is_rr, u16 cap_dep)
> +{
> + if (is_rr) { /* could only pick -(N*)-> */
> + if (cap_dep & DEP_NN_MASK)
> + return 0;
> + else if (cap_dep & DEP_NR_MASK)
> + return 1;
> + else
> + return -1;
> + } else {
> + if (cap_dep & DEP_NN_MASK || cap_dep & DEP_RN_MASK)

if (cap_dep & (DEP_NN_MASK | DEP_RN_MASK))

> + return 0;
> + else
> + return 1;
> + }
> +}

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;

entry->have_xr = is_xr(entry->dep);

Which to me is a much simpler construct, hmm?

> +
> visit_lock_entry(entry, lock);
> if (match(entry, data)) {
> *target_entry = entry;