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

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Date:	Fri, 28 Aug 2015 22:16:02 +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

On Fri, Aug 28, 2015 at 08:06:14PM +0800, Boqun Feng wrote:
> 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.

Ah.. just read through the thread you mentioned, I might misunderstand
you, probably because I didn't understand RCpc well..

You are saying that in a RELEASE we -might- switch from smp_lwsync() to
smp_mb() semantically, right? I guess this means we -might- switch from
RCpc to RCsc, right?

If so, I think I'd better to wait until we have a conclusion for this.

Thank you for your comments!

Regards,
Boqun


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