linux-kernel - Re: [PATCH v3 4/6] powerpc: atomic: Implement atomic{,64}_*_return

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Message-ID: <20151014010029.GB14305@fixme-laptop.cn.ibm.com>
Date:	Wed, 14 Oct 2015 09:00:29 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Peter Zijlstra <peterz@...radead.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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v3 4/6] powerpc: atomic: Implement atomic{,64}_*_return_*
 variants

On Tue, Oct 13, 2015 at 09:35:54PM +0800, Boqun Feng wrote:
> On Tue, Oct 13, 2015 at 02:21:32PM +0100, Will Deacon wrote:
> > On Mon, Oct 12, 2015 at 10:14:04PM +0800, Boqun Feng wrote:
> [snip]
> > > +/*
> > > + * 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")
> > 
> > I'm not keen on this barrier, as it sounds like it's part of the kernel
> > memory model, as opposed to an implementation detail on PowerPC (and
> > we've already got enough of that in the generic code ;).
> > 
> 
> Indeed, but we still have smp_lwsync() ;-)
> 
> > Can you name it something different please (and maybe #undef it when
> > you're done)?
> > 
> 
> I've considered #undef it after used, but now I think open code this
> into __atomic_op_acquire() of PPC is a better idea?
> 
> 
> #define __atomic_op_acquire(op, args...)				\
> ({									\
> 	typeof(op##_relaxed(args)) __ret  = op##_relaxed(args);		\
> 	__asm__ __volatile__(PPC_ACQUIRE_BARRIER : : : "memory");	\

Should be:
	__asm__ __volatile__(PPC_ACQUIRE_BARRIER "" : : : "memory");

> 	__ret;								\
> })
> 
> PPC_ACQUIRE_BARRIER will be empty if !SMP, so that will become a pure
> compiler barrier and just what we need.
> 
> Regards,
> Boqun

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