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Date: Fri, 7 Feb 2014 13:28:37 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Torsten Duwe <duwe@lst.de>
Subject: Re: [PATCH] Convert powerpc simple spinlocks into ticket locks
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Cc: Tom Musta <tommusta@gmail.com>, linux-kernel@vger.kernel.org,
 Paul Mackerras <paulus@samba.org>, Anton Blanchard <anton@samba.org>,
 Scott Wood <scottwood@freescale.com>,
 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, linuxppc-dev@lists.ozlabs.org,
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On Fri, Feb 07, 2014 at 12:49:49PM +0100, Torsten Duwe wrote:
> 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.

Right you are, add immediate it is.

> Besides, unlock increments the head.

No, unlock increments the tail, lock increments the head and waits until
the tail matches the pre-inc value.

That said, why do the atomic_inc() primitives do an carry add? (that's
where I borrowed it from).

> > > 	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.

spinlock_t should, ideally, be 32bits.

> What about
> 
> atomic_t tail;
> volatile int head; ?
> 
> Admittedly, that's usually 8 bytes instead of 4...

That still won't straddle a cacheline unless you do weird alignement
things which will bloat all the various data structures more still.

Anyway, you can do a version with lwarx/stwcx if you're looking get rid
of lharx.