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

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Message-ID: <20170518173010.GK21359@arm.com>
Date:   Thu, 18 May 2017 18:30:10 +0100
From:   Will Deacon <will.deacon@....com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     tglx@...utronix.de, 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>,
        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>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Chris Zankel <chris@...kel.net>,
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        linux-alpha@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
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        linux-arch@...r.kernel.org
Subject: Re: [PATCH 1/1] futex: remove duplicated code

On Wed, May 17, 2017 at 10:01:29AM +0200, Jiri Slaby wrote:
> On 05/15/2017, 03:16 PM, Will Deacon wrote:
> > Whilst I think this is a good idea, the code in question actually results
> > in undefined behaviour per the C spec and is reported by UBSAN.
> 
> Hi, yes, I know -- this patch was the 1st from the series of 3 which I
> sent a long time ago to fix that up too. But I remember your patch, so I
> sent only this one this time.
> 
> > See my
> > patch fixing arm64 here (which I'd forgotten about):
> > 
> > https://www.spinics.net/lists/linux-arch/msg38564.html
> > 
> > But, as stated in the thread above, I think we should go a step further
> > and remove FUTEX_OP_{OR,ANDN,XOR,OPARG_SHIFT} altogether. They don't
> > appear to be used by userspace, and this whole thing is a total mess.
> > 
> > Any thoughts?
> 
> Ok, I am all for that. I think the only question is who is going to do
> the work and submit it :)? Do I understand correctly to eliminate all
> these functions and the path into the kernel? But won't this break API
> -- are there really no users of this interface?

That's the million-dollar question, really. I don't know of any code using
it, and I couldn't find any when I looked (also nothing reported by Debian
Codesearch afaict), but I was hoping linux-arch might have some thoughts
on this too.

For now, I'll queue my arm64 patch before I forget about it again!

Will