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

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Message-ID: <20140207114949.GA2107@lst.de>
Date:	Fri, 7 Feb 2014 12:49:49 +0100
From:	Torsten Duwe <duwe@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Scott Wood <scottwood@...escale.com>, linux-kernel@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Tom Musta <tommusta@...il.com>,
	"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 11:45:30AM +0100, Peter Zijlstra wrote:
> 
> That might need to be lhz too, I'm confused on all the load variants.

;-)

> > unlock:
> > 	lhz	%0, 0, &tail
> > 	addic	%0, %0, 1

No carry with this one, I'd say.
Besides, unlock increments the head.

> > 	lwsync
> > 	sth	%0, 0, &tail
> > 

Given the beauty and simplicity of this, may I ask Ingo:
you signed off 314cdbefd1fd0a7acf3780e9628465b77ea6a836;
can you explain why head and tail must live on the same cache
line? Or is it just a space saver? I just ported it to ppc,
I didn't think about alternatives.

What about

atomic_t tail;
volatile int head; ?

Admittedly, that's usually 8 bytes instead of 4...

	Torsten

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