linux-kernel - Re: Preemptable Ticket Spinlock

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Date:	Mon, 22 Apr 2013 21:49:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Avi Kivity <avi.kivity@...il.com>,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Chegu Vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	Karen Noel <knoel@...hat.com>
Subject: Re: Preemptable Ticket Spinlock

On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote:
> On 04/22/2013 07:51 AM, Peter Zijlstra wrote:
> > On Sun, 2013-04-21 at 17:12 -0400, Rik van Riel wrote:
> >>
> >> If we always incremented the ticket number by 2 (instead of 1), then
> >> we could use the lower bit of the ticket number as the spinlock.
> >
> > ISTR that paravirt ticket locks already do that and use the lsb to
> > indicate the unlock needs to perform wakeups.
> >
> > Also, since all of this is virt nonsense, shouldn't it live in the
> > paravirt ticket lock code and leave the native code as is?
> 
> Sure, but that is still no reason not to have the virt
> implementation be as fast as possible, and share the same
> data type as the non-virt implementation.

It has to share the same data-type..

> Also, is it guaranteed that the native spin_lock code has
> not been called yet before we switch over to the paravirt
> functions?
> 
> If the native spin_lock code has been called already at
> that time, the native code would still need to be modified
> to increment the ticket number by 2, so we end up with a
> compatible value in each spin lock's .tickets field, and
> prevent a deadlock after we switch over to the paravirt
> variant.

I thought the stuff already made it upstream, but apparently not; the
lastest posting I'm aware of is here:

  https://lkml.org/lkml/2012/5/2/105

That stuff changes the normal ticket increment as well.. 

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