linux-kernel - Re: [RFC 2/5] atomics: introduce arch_atomic_op

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Date:	Fri, 28 Aug 2015 19:50:31 +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 2/5] atomics: introduce
 arch_atomic_op_{acquire,release,fence} helpers

Hi Peter,

On Fri, Aug 28, 2015 at 01:36:14PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 28, 2015 at 10:48:16AM +0800, Boqun Feng wrote:
> > Some architectures may have their special barriers for acquire, release
> > and fence semantics, general memory barriers(smp_mb__*_atomic()) in
> > __atomic_op_*() may be too strong, so arch_atomic_op_*() helpers are
> > introduced for architectures to provide their own version helpers to
> > build different variants based on _relaxed variants.
> > 
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > ---
> >  include/linux/atomic.h | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> > 
> > diff --git a/include/linux/atomic.h b/include/linux/atomic.h
> > index 00a5763..622255b 100644
> > --- a/include/linux/atomic.h
> > +++ b/include/linux/atomic.h
> > @@ -34,20 +34,33 @@
> >   * The idea here is to build acquire/release variants by adding explicit
> >   * barriers on top of the relaxed variant. In the case where the relaxed
> >   * variant is already fully ordered, no additional barriers are needed.
> > + *
> > + * Besides, if an arch has a special barrier for acquire/release, it could
> > + * implement its own arch_atomic_op_* and use the same framework for building
> > + * variants
> >   */
> > +#ifndef arch_atomic_op_acquire
> >  #define __atomic_op_acquire(op, args...)				\
> >  ({									\
> >  	typeof(op##_relaxed(args)) __ret  = op##_relaxed(args);		\
> >  	smp_mb__after_atomic();						\
> >  	__ret;								\
> >  })
> > +#else
> > +#define __atomic_op_acquire arch_atomic_op_acquire
> > +#endif
> 
> Not really a fan of this, its not consistent with the existing #ifndef
> guard style.

You suggestion is that I should:

#ifndef __atomic_op_acquire 
#define __atomic_op_acquire(op, args...)
({
	...
})
#endif

... and define powerpc specific __atomic_op_acquire in asm/atomic.h?

Regards,
Boqun

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