linux-kernel - Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

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Message-ID: <20140207161004.GD9987@twins.programming.kicks-ass.net>
Date:	Fri, 7 Feb 2014 17:10:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kumar Gala <galak@...nel.crashing.org>
Cc:	Torsten Duwe <duwe@....de>, Scott Wood <scottwood@...escale.com>,
	Tom Musta <tommusta@...il.com>, linux-kernel@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

On Fri, Feb 07, 2014 at 09:51:16AM -0600, Kumar Gala wrote:
> 
> On Feb 7, 2014, at 3:02 AM, Torsten Duwe <duwe@....de> wrote:
> 
> > On Thu, Feb 06, 2014 at 02:19:52PM -0600, Scott Wood wrote:
> >> On Thu, 2014-02-06 at 18:37 +0100, Torsten Duwe wrote:
> >>> On Thu, Feb 06, 2014 at 05:38:37PM +0100, Peter Zijlstra wrote:
> >> 
> >>>> Can you pair lwarx with sthcx ? I couldn't immediately find the answer
> >>>> in the PowerISA doc. If so I think you can do better by being able to
> >>>> atomically load both tickets but only storing the head without affecting
> >>>> the tail.
> > 
> > Can I simply write the half word, without a reservation, or will the HW caches
> > mess up the other half? Will it ruin the cache coherency on some (sub)architectures?
> 
> The coherency should be fine, I just can’t remember if you’ll lose the reservation by doing this.

It should; I suppose; seeing how you 'destroy' the state it got from the
load.

> >> Plus, sthcx doesn't exist on all PPC chips.
> > 
> > Which ones are lacking it? Do all have at least a simple 16-bit store?
> 
> Everything implements a simple 16-bit store, just not everything implements the store conditional of 16-bit data.

Ok, so then the last version I posted should work on those machines.
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