linux-kernel - Re: [thisops uV2 09/10] x86: this_cpu_cmpxchg and this_cpu_cmpxchg

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Date:	Mon, 29 Nov 2010 10:11:38 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [thisops uV2 09/10] x86: this_cpu_cmpxchg and this_cpu_cmpxchg_double
 operations

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.

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