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Date: Mon, 29 Nov 2010 10:11:38 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: akpm@linux-foundation.org, Pekka Enberg <penberg@cs.helsinki.fi>,
        linux-kernel@vger.kernel.org, Eric Dumazet <eric.dumazet@gmail.com>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [thisops uV2 09/10] x86: this_cpu_cmpxchg and this_cpu_cmpxchg_double
 operations
In-Reply-To: <20101127152026.GA15758@Krystal>
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References: <20101126210937.383047168@linux.com> <20101126210954.166783984@linux.com> <20101127152026.GA15758@Krystal>
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On Sat, 27 Nov 2010, Mathieu Desnoyers wrote:

> > + */
> > +#define percpu_cmpxchg16b(pcp, o1, o2, n1, n2)			\
> > +({									\
> > +	char __ret;							\
> > +	typeof(o1) __o1 = o1;						\
> > +	typeof(o1) __n1 = n1;						\
> > +	typeof(o2) __o2 = o2;						\
> > +	typeof(o2) __n2 = n2;						\
> > +	typeof(o2) __dummy;						\
> > +	VM_BUG_ON(((unsigned long)pcp) % 16);				\
>
> Restricting typing on "pcp" at build time might be more appropriate. E.g.
> creating a "struct doublecas" that would be forcefully aligned on 16 bytes. We
> could check that with __builtin_types_compatible_p and generate a build failure
> if necessary.

That would in turn restrict the types which can be used with
cmpxchg_double. The existing slub implementation already uses a mixture of
pointer and unsigned long. And there are 32 bit and 64 bit variants
already.