linux-kernel - Re: [RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return

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Message-ID: <20150828120614.GC29325@fixme-laptop.cn.ibm.com>
Date:	Fri, 28 Aug 2015 20:06:14 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>
Subject: Re: [RFC 3/5] powerpc: atomic: implement
 atomic{,64}_{add,sub}_return_* variants

Hi Peter,

On Fri, Aug 28, 2015 at 12:48:54PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 28, 2015 at 10:48:17AM +0800, Boqun Feng wrote:
> > +/*
> > + * Since {add,sub}_return_relaxed and xchg_relaxed are implemented with
> > + * a "bne-" instruction at the end, so an isync is enough as a acquire barrier
> > + * on the platform without lwsync.
> > + */
> > +#ifdef CONFIG_SMP
> > +#define smp_acquire_barrier__after_atomic() \
> > +	__asm__ __volatile__(PPC_ACQUIRE_BARRIER : : : "memory")
> > +#else
> > +#define smp_acquire_barrier__after_atomic() barrier()
> > +#endif
> > +#define arch_atomic_op_acquire(op, args...)				\
> > +({									\
> > +	typeof(op##_relaxed(args)) __ret  = op##_relaxed(args);		\
> > +	smp_acquire_barrier__after_atomic();				\
> > +	__ret;								\
> > +})
> > +
> > +#define arch_atomic_op_release(op, args...)				\
> > +({									\
> > +	smp_lwsync();							\
> > +	op##_relaxed(args);						\
> > +})
> 
> Urgh, so this is RCpc. We were trying to get rid of that if possible.
> Lets wait until that's settled before introducing more of it.
> 
> lkml.kernel.org/r/20150820155604.GB24100@....com

OK, get it. Thanks.

So I'm not going to introduce these arch specific macros, I think what I
need to implement are just _relaxed variants and cmpxchg_acquire.

Regards,
Boqun


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