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

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Date:	Fri, 7 Feb 2014 18:08:45 +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 04:18:47PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 07, 2014 at 01:28:37PM +0100, Peter Zijlstra wrote:
> > Anyway, you can do a version with lwarx/stwcx if you're looking get rid
> > of lharx.
> 
> the below seems to compile into relatively ok asm. It can be done better
> if you write the entire thing by hand though.

[...]

> 
> static inline unsigned int xadd(unsigned int *v, unsigned int i)
> {
> 	int t, ret;
> 	
> 	__asm__ __volatile__ (
> "1:	lwarx	%0, 0, %4\n"
> "	mr	%1, %0\n"
> "	add	%0, %3, %0\n"
> "	stwcx.	%0, %0, %4\n"
> "	bne-	1b\n"
> 	: "=&r" (t), "=&r" (ret), "+m" (*v)
> 	: "r" (i), "r" (v)
> 	: "cc");
> 
> 	return ret;
> }
> 
I don't like this xadd thing -- it's so x86 ;)
x86 has its LOCK prefix, ppc has ll/sc.
That should be reflected somehow IMHO.

Maybe if xadd became mandatory for some kernel library.

> 
> void ticket_unlock(tickets_t *lock)
> {
> 	ticket_t tail = lock->tail + 1;
> 
> 	/*
> 	 * The store is save against the xadd for it will make the ll/sc fail
> 	 * and try again. Aside from that PowerISA guarantees single-copy
> 	 * atomicy for half-word writes.
> 	 *
> 	 * And since only the lock owner will ever write the tail, we're good.
> 	 */
> 	smp_store_release(&lock->tail, tail);
> }

Yeah, let's try that on top of v2 (just posted).
First, I want to see v2 work as nicely as v1 --
compiling a debug kernel takes a while...

	Torsten

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