Date: Wed, 1 Apr 2015 20:48:59 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
        paolo.bonzini@gmail.com, konrad.wilk@oracle.com,
        boris.ostrovsky@oracle.com, paulmck@linux.vnet.ibm.com,
        riel@redhat.com, torvalds@linux-foundation.org,
        raghavendra.kt@linux.vnet.ibm.com, david.vrabel@citrix.com,
        oleg@redhat.com, scott.norton@hp.com, doug.hatch@hp.com,
        linux-arch@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
        luto@amacapital.net
Subject: Re: [PATCH 8/9] qspinlock: Generic paravirt support
Message-ID: <20150401184858.GA9791@dyad.arnhem.chello.nl>
References: <20150316131613.720617163@infradead.org>
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 <5509E51D.7040909@hp.com>
 <20150319101242.GM21418@twins.programming.kicks-ass.net>
 <20150319122536.GD11574@worktop.ger.corp.intel.com>
 <551C1ACE.4090408@hp.com>
 <20150401171223.GO23123@twins.programming.kicks-ass.net>
 <20150401174239.GO24151@twins.programming.kicks-ass.net>
 <20150401181744.GE32047@worktop.ger.corp.intel.com>
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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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