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

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Message-ID: <20150401174239.GO24151@twins.programming.kicks-ass.net>
Date:	Wed, 1 Apr 2015 19:42:39 +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 07:12:23PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 01, 2015 at 12:20:30PM -0400, Waiman Long wrote:
> > After more careful reading, I think the assumption that the presence of an
> > unused bucket means there is no match is not true. Consider the scenario:
> > 
> > 1. cpu 0 puts lock1 into hb[0]
> > 2. cpu 1 puts lock2 into hb[1]
> > 3. cpu 2 clears hb[0]
> > 4. cpu 3 looks for lock2 and doesn't find it
> 
> Hmm, yes. The only way I can see that being true is if we assume entries
> are never taken out again.
> 
> The wikipedia page could use some clarification here, this is not clear.
> 
> > At this point, I am thinking using back your previous idea of passing the
> > queue head information down the queue.
> 
> Having to scan the entire array for a lookup sure sucks, but the wait
> loops involved in the other idea can get us in the exact predicament we
> were trying to get out, because their forward progress depends on other
> CPUs.
> 
> 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.

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