linux-kernel - Re: [PATCH 8/9] qspinlock: Generic paravirt support

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Message-ID: <20150401184858.GA9791@dyad.arnhem.chello.nl>
Date:	Wed, 1 Apr 2015 20:48:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	paolo.bonzini@...il.com, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
	riel@...hat.com, torvalds@...ux-foundation.org,
	raghavendra.kt@...ux.vnet.ibm.com, david.vrabel@...rix.com,
	oleg@...hat.com, scott.norton@...com, doug.hatch@...com,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	luto@...capital.net
Subject: Re: [PATCH 8/9] qspinlock: Generic paravirt support

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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