MIME-Version: 1.0
In-Reply-To: <3662dd60-2467-f858-dc32-0b0fb6abb33b@suse.cz>
References: <20170303122712.13353-1-jslaby@suse.cz> <20170304130550.GT21222@n2100.armlinux.org.uk>
 <3994975e-89a5-d2b5-60be-a8633ddc3733@zytor.com> <20170304213805.GA2449@lianli.shorne-pla.net>
 <201703042308.v24N8wvh012716@mail.zytor.com> <20170304233919.GB2449@lianli.shorne-pla.net>
 <3662dd60-2467-f858-dc32-0b0fb6abb33b@suse.cz>
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Mon, 6 Mar 2017 15:14:08 +0100
Message-ID: <CAMuHMdVgechGXW6JkY2j=ka8-YZY_paoVDuHiAj66B5AP5nP9A@mail.gmail.com>
Subject: Re: [PATCH 1/3] futex: remove duplicated code
To: Jiri Slaby <jslaby@suse.cz>
Cc: Stafford Horne <shorne@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>, Vineet Gupta <vgupta@synopsys.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, Richard Kuo <rkuo@codeaurora.org>,
        Tony Luck <tony.luck@intel.com>, Fenghua Yu <fenghua.yu@intel.com>,
        Michal Simek <monstr@monstr.eu>, Ralf Baechle <ralf@linux-mips.org>,
        Jonas Bonn <jonas@southpole.se>,
        Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
        "James E.J. Bottomley" <jejb@parisc-linux.org>,
        Helge Deller <deller@gmx.de>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Rich Felker <dalias@libc.org>, DavidS.Miller@zytor.com
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1764
Lines: 46

Hi Jiri,

On Mon, Mar 6, 2017 at 9:46 AM, Jiri Slaby <jslaby@suse.cz> wrote:
>     futex: make the encoded_op decoding readable
>
>     Decoding of encoded_op is a bit unreadable. It contains shifts to the
>     left and to the right by some constants. Make it clearly visible what
>     part of the bit mask is taken and shift the values only to the right
>     appropriately. And make sure sign extension takes place using
>     sign_extend32.
>
>     Signed-off-by: Jiri Slaby <jslaby@suse.cz>
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 0ead0756a593..f90314bd42cb 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1461,10 +1461,10 @@ futex_wake(u32 __user *uaddr, unsigned int
> flags, int nr_wake, u32 bitset)
>
>  static int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
>  {
> -       int op = (encoded_op >> 28) & 7;
> -       int cmp = (encoded_op >> 24) & 15;

At least for the two above (modulo 7 vs 15?), the old decoding code matched
the flow of operation in FUTEX_OP().

> -       int oparg = (encoded_op << 8) >> 20;
> -       int cmparg = (encoded_op << 20) >> 20;
> +       int op =                  (encoded_op & 0x70000000) >> 28;
> +       int cmp =                 (encoded_op & 0x0f000000) >> 24;
> +       int oparg = sign_extend32((encoded_op & 0x00fff000) >> 12, 12);
> +       int cmparg = sign_extend32(encoded_op & 0x00000fff, 12);
>         int oldval, ret;

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds