On Wed, Apr 20, 2016 at 10:15:09PM +0800, Pan Xinhui wrote:
> >> +static struct pv_node *pv_lookup_hash(struct qspinlock *lock)
> >> +{
> >> + unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
> >> + struct pv_hash_entry *he;
> >> +
> >> + for_each_hash_entry(he, offset, hash) {
> >> + struct qspinlock *l = READ_ONCE(he->lock);
> >> +
> >> + if (l == lock)
> >
> > The other loop writes:
> >
> > if (READ_ONCE(he->lock) == lock)
> >
> Maybe because we check l is NULL or not later. So save one load.
Ah duh, yes.
> >> + return READ_ONCE(he->node);
> >> + /*
> >> + * Presence of an empty slot signal the end of search. We
> >> + * may miss the entry, but that will limit the amount of
> >> + * time doing the search when the desired entry isn't there.
> >> + */
> >> + else if (!l)
> >> + break;
> >
> > That 'else' is entirely pointless. Also, why isn't this: return NULL;
> >
> >> + }
> >> + return NULL;
> >
> > and this BUG() ?
> >
> It's not a bug, the lock might not be stored in the hashtable. in unlock
> function, we will unhash the lock, then what will happen is:
It should be if the above becomes a return NULL, no?
If we can iterate the _entire_ hashtable, this lookup can be immensely
expensive and we should not be doing it inside of a wait-loop.
