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

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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))))
> 

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[PATCH 1/3] futex: remove duplicated code Jiri Slaby <jslaby@suse.cz> - 2017-03-03 13:40 +0100
  [PATCH 2/3] futex: fix decoding of operation Jiri Slaby <jslaby@suse.cz> - 2017-03-03 19:30 +0100
  [PATCH 3/3] futex: make the encoded_op decoding readable Jiri Slaby <jslaby@suse.cz> - 2017-03-03 19:30 +0100
    Re: [PATCH 3/3] futex: make the encoded_op decoding readable Jiri Slaby <jslaby@suse.cz> - 2017-03-05 09:00 +0100
  Re: [PATCH 1/3] futex: remove duplicated code Heiko Carstens <heiko.carstens@de.ibm.com> - 2017-03-03 21:10 +0100
  Re: [PATCH 1/3] futex: remove duplicated code Michael Ellerman <mpe@ellerman.id.au> - 2017-03-04 14:00 +0100
  Re: [PATCH 1/3] futex: remove duplicated code Russell King - ARM Linux <linux@armlinux.org.uk> - 2017-03-04 14:20 +0100
    Re: [PATCH 1/3] futex: remove duplicated code "H. Peter Anvin" <hpa@zytor.com> - 2017-03-04 20:30 +0100
      Re: [PATCH 1/3] futex: remove duplicated code Stafford Horne <shorne@gmail.com> - 2017-03-04 22:40 +0100
        Re: [PATCH 1/3] futex: remove duplicated code "H. Peter Anvin" <hpa@zytor.com> - 2017-03-05 00:20 +0100
          Re: [PATCH 1/3] futex: remove duplicated code Stafford Horne <shorne@gmail.com> - 2017-03-05 00:40 +0100
            Re: [PATCH 1/3] futex: remove duplicated code Jiri Slaby <jslaby@suse.cz> - 2017-03-06 09:50 +0100
              Re: [PATCH 1/3] futex: remove duplicated code "H. Peter Anvin" <hpa@zytor.com> - 2017-03-06 10:20 +0100
              Re: [PATCH 1/3] futex: remove duplicated code Geert Uytterhoeven <geert@linux-m68k.org> - 2017-03-06 15:20 +0100
        Re: [PATCH 1/3] futex: remove duplicated code "H. Peter Anvin" <hpa@zytor.com> - 2017-03-05 00:20 +0100
    Re: [PATCH 1/3] futex: remove duplicated code Rob Landley <rob@landley.net> - 2017-03-09 05:20 +0100
      Re: [PATCH 1/3] futex: remove duplicated code "H. Peter Anvin" <hpa@zytor.com> - 2017-03-09 06:10 +0100
        Re: [PATCH 1/3] futex: remove duplicated code Rich Felker <dalias@libc.org> - 2017-03-09 23:50 +0100
  Re: [PATCH 1/3] futex: remove duplicated code Rich Felker <dalias@libc.org> - 2017-03-06 03:20 +0100
  Re: [PATCH 1/3] futex: remove duplicated code Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2017-03-09 23:50 +0100

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