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Date: Wed, 20 Apr 2016 23:05:21 +0800 From: Pan Xinhui <xinhui@...ux.vnet.ibm.com> To: Peter Zijlstra <peterz@...radead.org> CC: Waiman Long <Waiman.Long@....com>, Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@....com>, Douglas Hatch <doug.hatch@....com> Subject: Re: [PATCH v2] locking/pvqspinlock: Add lock holder CPU argument to pv_wait() On 2016年04月20日 22:18, Peter Zijlstra wrote: > 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? > no, the lock might not be there, even if we search the whole hashtable. Only pv_kick_node and pv_wait_head_or_lock will hash the lock. if both vcpu's state is vcpu_running, who will hash the lock on behalf of us? Can pv_wait return without anyone kicking it? If yes, then this not a bug. > 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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