linux-kernel - Re: [thisops uV2 04/10] x86: Support for this_cpu_add,sub,dec,inc

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Date:	Mon, 29 Nov 2010 12:54:42 -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 04/10] x86: Support for
 this_cpu_add,sub,dec,inc_return

On Mon, 29 Nov 2010, Mathieu Desnoyers wrote:

> > > > +#define percpu_add_return_op(var, val)					\
> > > > +({									\
> > > > +	typedef typeof(var) pao_T__;					\
> > > > +	typeof(var) pfo_ret__ = val;					\
> > > > +	if (0) {							\
> > > > +		pao_T__ pao_tmp__;					\
> > > > +		pao_tmp__ = (val);					\
> > > > +		(void)pao_tmp__;					\
> > > > +	}								\
> > >
> > > OK, I'm dumb: why is the above needed ?
> >
> > Ensure that the compiler agrees that  *var and val are compatible. Taken
> > over from percpu_add_op().
>
> Isn't that the purpose of __builtin_types_compatible_p(t1, t2) ?

We also have a __same_type() macro in linux/compiler.h... But if I use
that the kernel build fails. I guess the check is too strict.

---
 arch/x86/include/asm/percpu.h |    6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

Index: linux-2.6/arch/x86/include/asm/percpu.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/percpu.h	2010-11-29 12:46:50.000000000 -0600
+++ linux-2.6/arch/x86/include/asm/percpu.h	2010-11-29 12:52:25.000000000 -0600
@@ -127,11 +127,7 @@ do {									\
 	typedef typeof(var) pao_T__;					\
 	const int pao_ID__ = (__builtin_constant_p(val) &&		\
 			      ((val) == 1 || (val) == -1)) ? (val) : 0;	\
-	if (0) {							\
-		pao_T__ pao_tmp__;					\
-		pao_tmp__ = (val);					\
-		(void)pao_tmp__;					\
-	}								\
+	BUILD_BUG_ON(!__same_type(var, val));				\
 	switch (sizeof(var)) {						\
 	case 1:								\
 		if (pao_ID__ == 1)					\


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