Messages in this thread | | | Date | Wed, 20 Apr 2016 16:18:19 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] locking/pvqspinlock: Add lock holder CPU argument to pv_wait() |
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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.
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