Re: [Xen-devel] [PATCH 8/9] qspinlock: Generic paravirt support

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Wed, Apr 01, 2015 at 02:54:45PM -0400, Waiman Long wrote:
> On 04/01/2015 02:17 PM, Peter Zijlstra wrote:
> >On Wed, Apr 01, 2015 at 07:42:39PM +0200, Peter Zijlstra wrote:
> >>>Hohumm.. time to think more I think ;-)
> >>So bear with me, I've not really pondered this well so it could be full
> >>of holes (again).
> >>
> >>After the cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL) succeeds the
> >>spin_unlock() must do the hash lookup, right? We can make the lookup
> >>unhash.
> >>
> >>If the cmpxchg() fails the unlock will not do the lookup and we must
> >>unhash.
> >The idea being that the result is that any lookup is guaranteed to find
> >an entry, which reduces our worst case lookup cost to whatever the worst
> >case insertion cost was.
> >
> 
> I think it doesn't matter who did the unhashing. Multiple independent locks
> can be hashed to the same value. Since they can be unhashed independently,
> there is no way to know whether you have checked all the possible buckets.

oh but the crux is that you guarantee a lookup will find an entry. it will
never need to iterate the entire array.

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