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

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Message-ID: <20140207082437.GA26811@lst.de>
Date:	Fri, 7 Feb 2014 09:24:37 +0100
From:	Torsten Duwe <duwe@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

On Thu, Feb 06, 2014 at 07:08:26PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 06, 2014 at 06:37:27PM +0100, Torsten Duwe wrote:
> > I must admit that I haven't tested the patch on non-pseries ppc64 nor on
> > ppc32. Only ppc64 has the ldarx and I tried to atomically replace the 
> > holder along with the locks. That might prove unneccessary.
> 
> But what is the holder for? Can't we do away with that field?

Scott, Peter: good questions.
The conditional is wrong because I confused pSeries with ppc64 CPUs with
64-bit kernels. I got deluded by the LOCK_TOKEN definition above. Is that
correctly ifdef'd, with PPC64? The holder field should be ifdef'd
CONFIG_PPC_SPLPAR, independent of ppc64.

It is an advisory performance hint, and doesn't need to be updated atomically
with the lock; this and the above are 2 reasons to drop the asm string
operand size voodoo as well.

Thanks,
	Torsten

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