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

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Date:   Fri, 26 May 2017 08:54:31 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Will Deacon <will.deacon@....com>
cc:     Jiri Slaby <jslaby@...e.cz>, 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>,
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Subject: Re: [PATCH 1/1] futex: remove duplicated code

On Thu, 25 May 2017, Will Deacon wrote:
> On Mon, May 22, 2017 at 11:11:33PM +0200, Thomas Gleixner wrote:
> > On Mon, 15 May 2017, Will Deacon wrote:
> > > On Mon, May 15, 2017 at 03:07:42PM +0200, 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).
> > > 
> > > 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. 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.
> > 
> > You wish. The constants are not used, but FUTEX_WAKE_OP _IS_ used by
> > glibc. They only have one argument it seems:
> > 
> >    #define FUTEX_OP_CLEAR_WAKE_IF_GT_ONE      ((4 << 24) | 1)
> > 
> > but I'm pretty sure that there is enough (probably horrible) code (think
> > java) out there using FUTEX_WAKE_OP for whatever (non)sensical reasons in
> > any available combination.
> 
> Indeed, and I'm not proposing to get rid of that. It's the grossly
> over-engineered array of operations and the FUTEX_OP_OPARG_SHIFT modifier
> that I think we should kill. The latter likely behaves differently across
> different architectures and potentially depending on the toolchain you used
> to build the kernel.
> 
> Does anybody know the history behind the interface design?

Which design?

4732efbeb997 ("[PATCH] FUTEX_WAKE_OP: pthread_cond_signal() speedup")

Thanks,

	tglx