On Tue, Jan 17, 2017 at 02:24:08PM +0800, Boqun Feng wrote: > 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.
Thank you for confirming it. > > 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); > > >
