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

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Date:	Tue, 13 Oct 2015 21:35:54 +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 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");	\
	__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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