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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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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