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

From: Thomas Gleixner <tglx@linutronix.de>
To: Will Deacon <will.deacon@arm.com>
Cc: Jiri Slaby <jslaby@suse.cz>,
	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>,
	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>,
	Stafford Horne <shorne@gmail.com>,
	"James E.J. Bottomley" <jejb@parisc-linux.org>,
	Helge Deller <deller@gmx.de>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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	Rich Felker <dalias@libc.org>,
	"David S. Miller" <davem@davemloft.net>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH 1/1] futex: remove duplicated code
Date: Fri, 26 May 2017 08:54:31 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.20.1705260849520.1902@nanos> (raw)
In-Reply-To: <20170525142829.GO2859@arm.com>

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