linux-kernel - Re: next build: 235 warnings 3 failures (next/next-20151117)

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Message-ID: <20151118153634.GH1588@arm.com>
Date:	Wed, 18 Nov 2015 15:36:34 +0000
From:	Will Deacon <will.deacon@....com>
To:	David Laight <David.Laight@...lab.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <catalin.marinas@....com>,
	"kernel-build-reports@...ts.linaro.org" 
	<kernel-build-reports@...ts.linaro.org>,
	"olof@...om.net" <olof@...om.net>,
	David Miller <davem@...emloft.net>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: next build: 235 warnings 3 failures (next/next-20151117)

On Wed, Nov 18, 2015 at 03:21:19PM +0000, David Laight wrote:
> From: Will Deacon
> > Sent: 18 November 2015 12:28
> > On Wed, Nov 18, 2015 at 12:11:25PM +0000, David Laight wrote:
> > > From: Will Deacon
> > > >   http://lists.infradead.org/pipermail/linux-arm-kernel/2015-November/386094.html
> > >
> > > That patch forces a memory write-read and returns uninitialised stack
> > > for short reads.
> > 
> > Really? The disassembly looks fine to me. Do you have a concrete example
> > of where you think it goes wrong, please?
> > 
> > > Who knows what happens on big-endian systems.
> > 
> > The same thing as READ_ONCE? I'll test it there to make sure, but I
> > don't see a problem.
> 
> Ah, god, it is absolutely horrid. But probably right :-(

Yeah, I wasn't pretending it was nice :) FWIW, I've given it a reasonable
testing in both little-endian and big-endian configurations and it seems
to be happy.

> Do all the lda variants zero extend to 64 bits ?

Yes.

> If so maybe you could use a single 64 bit variable for the result of the read
> and then cast it to typeof(*p) to get the required sign extension for
> small integer types.

That was the original proposal from Arnd, but I want this to work with
structures smaller than 64-bit (e.g. arch_spinlock_t), so that's why
I decided to follow the approach laid down by READ_ONCE.

Will
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