Re: [PATCH v4 07/15] lockdep: Implement crossrelease feature

Byungchul Park
2017-01-17T02:33:51Z

On Mon, Jan 16, 2017 at 04:13:19PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 09, 2016 at 02:12:03PM +0900, Byungchul Park wrote:
> > + /*
> > + * We assign class_idx here redundantly even though following
> > + * memcpy will cover it, in order to ensure a rcu reader can
> > + * access the class_idx atomically without lock.
> > + *
> > + * Here we assume setting a word-sized variable is atomic.
>
> which one, where?

I meant xlock_class(xlock) in check_add_plock().

I was not sure about the following two.

1. Is it ordered between following a and b?
a. memcpy -> list_add_tail_rcu
b. list_for_each_entry_rcu -> load class_idx (xlock_class)
I assumed that it's not ordered.
2. Does memcpy guarantee atomic store for each word?
I assumed that it doesn't.

But I think I was wrong.. The first might be ordered. I will remove
the following redundant statement. It'd be orderd, right?

>
> > + */
> > + xlock->hlock.class_idx = hlock->class_idx;
> > + gen_id = (unsigned int)atomic_inc_return(&cross_gen_id);
> > + WRITE_ONCE(xlock->gen_id, gen_id);
> > + memcpy(&xlock->hlock, hlock, sizeof(struct held_lock));
> > + INIT_LIST_HEAD(&xlock->xlock_entry);
> > + list_add_tail_rcu(&xlock->xlock_entry, &xlocks_head);
>