linux-kernel - Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values

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Message-ID: <xa1tmvwnfg4u.fsf@mina86.com>
Date:	Wed, 16 Sep 2015 00:54:09 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Steven Rostedt \(Red Hat\)" <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Dave Chinner <dchinner@...hat.com>,
	Joe Perches <joe@...ches.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values

On Tue, Sep 15 2015, Andrew Morton wrote:
> On Mon, 14 Sep 2015 23:46:32 -0400 Tejun Heo <tj@...nel.org> wrote:
>
>> Anyways, let's please get abs() working for all types, one way or the
>> other.
>
> That would be by far the best solution, of course.
>
> This seems to work OK:
>
> --- a/include/linux/kernel.h~a
> +++ a/include/linux/kernel.h
> @@ -207,8 +207,11 @@ extern int _cond_resched(void);
>   * for those.
>   */
>  #define abs(x) ({						\
> -		long ret;					\
> -		if (sizeof(x) == sizeof(long)) {		\
> +		s64 ret;					\
> +		if (sizeof(x) == sizeof(s64)) {			\
> +			s64 __x = (x);				\
> +			ret = (__x < 0) ? -__x : __x;		\
> +		} else if (sizeof(x) == sizeof(long)) {		\
>  			long __x = (x);				\
>  			ret = (__x < 0) ? -__x : __x;		\
>  		} else {					\

If the return type is an issue, we can use __builtin_choose_expr, no?

#define abs(x) __builtin_choose_expr(sizeof(x) == sizeof(s64), abs64(x), ({ \
		long ret;					\
		if (sizeof(x) == sizeof(long)) {		\
			long __x = (x);				\
			ret = (__x < 0) ? -__x : __x;		\
		} else {					\
			int __x = (x);				\
			ret = (__x < 0) ? -__x : __x;		\
		}						\
		ret;						\
	}))

This is awkward but will make even printk happy.

>
> Test case:
>
> --- /dev/null
> +++ a/lib/xx.c
> @@ -0,0 +1,33 @@
> +#include <linux/kernel.h>
> +
> +#define newabs(x) ({						\
> +		s64 ret;					\
> +		if (sizeof(x) == sizeof(s64)) {			\
> +			s64 __x = (x);				\
> +			ret = (__x < 0) ? -__x : __x;		\
> +		} else if (sizeof(x) == sizeof(long)) {		\
> +			long __x = (x);				\
> +			ret = (__x < 0) ? -__x : __x;		\
> +		} else {					\
> +			int __x = (x);				\
> +			ret = (__x < 0) ? -__x : __x;		\
> +		}						\
> +		ret;						\
> +	})
> +
> +#define oldabs(x) ({						\
> +		long ret;					\
> +		if (sizeof(x) == sizeof(long)) {		\
> +			long __x = (x);				\
> +			ret = (__x < 0) ? -__x : __x;		\
> +		} else {					\
> +			int __x = (x);				\
> +			ret = (__x < 0) ? -__x : __x;		\
> +		}						\
> +		ret;						\
> +	})
> +
> +int foo(int x)
> +{
> +	return oldabs(x);
> +}
> diff -puN lib/Makefile~b lib/Makefile
> --- a/lib/Makefile~b
> +++ a/lib/Makefile
> @@ -13,7 +13,7 @@ lib-y := ctype.o string.o vsprintf.o cmd
>  	 sha1.o md5.o irq_regs.o argv_split.o \
>  	 proportions.o flex_proportions.o ratelimit.o show_mem.o \
>  	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
> -	 earlycpio.o seq_buf.o nmi_backtrace.o
> +	 earlycpio.o seq_buf.o nmi_backtrace.o xx.o
>  
>  obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o
>  lib-$(CONFIG_MMU) += ioremap.o
>
>
> on i386, xx.o's text is 68 bytes with either newabs() or oldabs().
>
>
> lib/percpu_counter.o's text does get larger with newabs().  That's
> because __percpu_counter_compare() is doing abs() on an s64, doh.
>

-- 
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.o. | Liege of Serenely Enlightened Majesty of         o' \,=./ `o
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