linux-kernel - Re: [PATCH 1/3] futex: remove duplicated code

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Message-ID: <20170304213805.GA2449@lianli.shorne-pla.net>
Date:   Sun, 5 Mar 2017 06:38:05 +0900
From:   Stafford Horne <shorne@...il.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Jiri Slaby <jslaby@...e.cz>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Richard Kuo <rkuo@...eaurora.org>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Michal Simek <monstr@...str.eu>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Deller <deller@....de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
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        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
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Subject: Re: [PATCH 1/3] futex: remove duplicated code

On Sat, Mar 04, 2017 at 11:15:17AM -0800, H. Peter Anvin wrote:
> On 03/04/17 05:05, Russell King - ARM Linux wrote:
> >>  
> >> +static int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
> >> +{
> >> +	int op = (encoded_op >> 28) & 7;
> >> +	int cmp = (encoded_op >> 24) & 15;
> >> +	int oparg = (encoded_op << 8) >> 20;
> >> +	int cmparg = (encoded_op << 20) >> 20;
> > 
> > Hmm.  oparg and cmparg look like they're doing these shifts to get sign
> > extension of the 12-bit values by assuming that "int" is 32-bit -
> > probably worth a comment, or for safety, they should be "s32" so it's
> > not dependent on the bit-width of "int".
> > 
> 
> For readability, perhaps we should make sign- and zero-extension an
> explicit facility?

There is some of this in already here, 32 and 64 bit versions:

  include/linux/bitops.h

Do we really need zero extension? It seems the same.

Example implementation from bitops.h

static inline __s32 sign_extend32(__u32 value, int index)
{
        __u8 shift = 31 - index;
        return (__s32)(value << shift) >> shift;
}

> /*
>  * Truncate an integer x to n bits, using sign- or
>  * zero-extension, respectively.
>  */
> static inline __const_func__ s32 sex32(s32 x, int n)
> {
>   return (x << (32-n)) >> (32-n);
> }
> 
> static inline __const_func__ s64 sex64(s64 x, int n)
> {
>   return (x << (64-n)) >> (64-n);
> }
> 
> #define sex(x,y)						\
> 	((__typeof__(x))					\
> 	 (((__builtin_constant_p(y) && ((y) <= 32)) ||		\
> 	   (sizeof(x) <= sizeof(s32)))				\
> 	  ? sex32((x),(y)) : sex64((x),(y))))
> 
> static inline __const_func__ u32 zex32(u32 x, int n)
> {
>   return (x << (32-n)) >> (32-n);
> }
> 
> static inline __const_func__ u64 zex64(u64 x, int n)
> {
>   return (x << (64-n)) >> (64-n);
> }
> 
> #define zex(x,y)						\
> 	((__typeof__(x))					\
> 	 (((__builtin_constant_p(y) && ((y) <= 32)) ||		\
> 	   (sizeof(x) <= sizeof(u32)))				\
> 	  ? zex32((x),(y)) : zex64((x),(y))))
>