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

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Message-ID: <20170306015207.GN1520@brightrain.aerifal.cx>
Date:   Sun, 5 Mar 2017 20:52:07 -0500
From:   Rich Felker <dalias@...c.org>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     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>,
        Russell King <linux@...linux.org.uk>,
        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>,
        Stafford Horne <shorne@...il.com>,
        "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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        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
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        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
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        linux-arch@...r.kernel.org
Subject: Re: [PATCH 1/3] futex: remove duplicated code

On Fri, Mar 03, 2017 at 01:27:10PM +0100, Jiri Slaby wrote:
> There is code duplicated over all architecture's headers for
> futex_atomic_op_inuser. Namely op decoding, access_ok check for uaddr,
> and comparison of the result.
> 
> Remove this duplication and leave up to the arches only the needed
> assembly which is now in arch_futex_atomic_op_inuser.
> 
> Note that s390 removed access_ok check in d12a29703 ("s390/uaccess:
> remove pointless access_ok() checks") as access_ok there returns true.
> We introduce it back to the helper for the sake of simplicity (it gets
> optimized away anyway).

Overall I'm in favor of this patch, and it's close to what I had in
mind in the commit message for
00b73d8d1b7131da03aec73011a7286f566fe87f. But I'd actually like to see
it go further. These ops are mainly (only?) used for the (almost never
used) FUTEX_WAKE_OP operation, and there's very little sense in trying
to optimize them with dedicated arch-specific forms like "lock xadd".
Instead the entire logic should be in an arch-generic file, and all
the arch should need to provide is a cmpxchg-on-user-memory primitive
for it to use. On most archs, the same cmpxchg used in kernelspace
should also work for user addresses, meaning a huge amount of
unmaintained, largely untested, junk code can be removed.

Rich