Date: Mon, 27 Aug 2007 17:11:40 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
To: Grant Grundler <grundler@parisc-linux.org>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       parisc-linux@parisc-linux.org, Christoph Lameter <clameter@sgi.com>
Subject: Re: [parisc-linux] [patch 15/23] Add cmpxchg_local to parisc
Message-ID: <20070827211140.GB10627@Krystal>
References: <20070812145434.520271946@polymtl.ca> <20070812145840.691277845@polymtl.ca> <20070827210432.GA22484@colo.lackof.org>
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* Grant Grundler (grundler@parisc-linux.org) wrote:
> On Sun, Aug 12, 2007 at 10:54:49AM -0400, Mathieu Desnoyers wrote:
> > Use the new generic cmpxchg_local (disables interrupt). Also use the generic
> > cmpxchg as fallback if SMP is not set.
> 
> Mathieu,
> thanks for adding __cmpxchg_local to parisc.... but why do we need it?
> 
> By definition, atomic operators are, well, atomic.
> 
> I searched for __cmpxchg_local and found this reference:
>     http://www.ussg.iu.edu/hypermail/linux/kernel/0612.2/1337.html
> 
> but the "root" of that thread (Dec 20, 2006):
>     http://www.ussg.iu.edu/hypermail/linux/kernel/0612.2/1334.html
> 
> Doesn't explain the difference between "local" and "non-local" either.
> Per CPU data should only need memory barriers (in some cases) and
> protection against interrupts (in probably more cases). So I'm not
> understanding why a new set of APIs is needed.
> 
> Can you add a description to Documentation/atomic_ops.txt ?
> *sigh* sorry for being "late to the party" on this one...
> 

Does Documentation/local_ops.txt answer your questions ? If not, please
tell me and I'll gladly explain more.

Mathieu

> cheers,
> grant
> 
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> > CC: clameter@sgi.com
> > CC: parisc-linux@parisc-linux.org
> > ---
> >  include/asm-parisc/atomic.h |   29 +++++++++++++++++++++++++++++
> >  1 file changed, 29 insertions(+)
> > 
> > Index: linux-2.6-lttng/include/asm-parisc/atomic.h
> > ===================================================================
> > --- linux-2.6-lttng.orig/include/asm-parisc/atomic.h	2007-07-20 19:44:40.000000000 -0400
> > +++ linux-2.6-lttng/include/asm-parisc/atomic.h	2007-07-20 19:44:47.000000000 -0400
> > @@ -122,6 +122,35 @@ __cmpxchg(volatile void *ptr, unsigned l
> >  				    (unsigned long)_n_, sizeof(*(ptr))); \
> >    })
> >  
> > +#include <asm-generic/cmpxchg-local.h>
> > +
> > +static inline unsigned long __cmpxchg_local(volatile void *ptr,
> > +				      unsigned long old,
> > +				      unsigned long new_, int size)
> > +{
> > +	switch (size) {
> > +#ifdef CONFIG_64BIT
> > +	case 8:	return __cmpxchg_u64((unsigned long *)ptr, old, new_);
> > +#endif
> > +	case 4:	return __cmpxchg_u32(ptr, old, new_);
> > +	default:
> > +		return __cmpxchg_local_generic(ptr, old, new_, size);
> > +	}
> > +}
> > +
> > +/*
> > + * cmpxchg_local and cmpxchg64_local are atomic wrt current CPU. Always make
> > + * them available.
> > + */
> > +#define cmpxchg_local(ptr,o,n)					  	\
> > +     (__typeof__(*(ptr)))__cmpxchg_local((ptr), (unsigned long)(o),	\
> > +			   	 (unsigned long)(n), sizeof(*(ptr)))
> > +#ifdef CONFIG_64BIT
> > +#define cmpxchg64_local(ptr,o,n) cmpxchg_local((ptr), (o), (n))
> > +#else
> > +#define cmpxchg64_local(ptr,o,n) __cmpxchg64_local_generic((ptr), (o), (n))
> > +#endif
> > +
> >  /* Note that we need not lock read accesses - aligned word writes/reads
> >   * are atomic, so a reader never sees unconsistent values.
> >   *
> > 
> > -- 
> > Mathieu Desnoyers
> > Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
> > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> > _______________________________________________
> > parisc-linux mailing list
> > parisc-linux@lists.parisc-linux.org
> > http://lists.parisc-linux.org/mailman/listinfo/parisc-linux

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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