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

Boqun Feng
2017-01-17T06:23:48Z

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On Tue, Jan 17, 2017 at 11:33:41AM +0900, Byungchul Park wrote:
> 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?
>

Yes, a and b are ordered, IOW, they could be paired, meaning when we
got the item in a list_for_each_entry_rcu() loop, all memory operations
before the corresponding list_add_tail_rcu() should be observed by us.

Regards,
Boqun

> >
> > > + */
> > > + 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);
> >

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