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

[Will Deacon <will.deacon@arm.com>]

On Wed, Nov 18, 2015 at 12:11:25PM +0000, David Laight wrote:
> From: Will Deacon
> > Sent: 18 November 2015 10:14
> > On Tue, Nov 17, 2015 at 08:17:17PM +0100, Arnd Bergmann wrote:
> > > On Tuesday 17 November 2015 17:12:37 Will Deacon wrote:
> > > > On Tue, Nov 17, 2015 at 06:03:40PM +0100, Arnd Bergmann wrote:
> > > > > On Tuesday 17 November 2015 16:44:53 Will Deacon wrote:
> > > > > > > 8<----
> > > > > > > Subject: ARM64: make smp_load_acquire() work with const arguments
> > > > > > >
> > > > > > > smp_load_acquire() uses typeof() to declare a local variable for temporarily
> > > > > > > storing the output of the memory access. This fails when the argument is
> > > > > > > constant, because the assembler complains about using a constant register
> > > > > > > as output:
> > > > > > >
> > > > > > >  arch/arm64/include/asm/barrier.h:71:3: error: read-only variable '___p1'
> > > > > > >  used as 'asm' output
> > > > > >
> > > > > > Do you know the usage in the kernel causing this warning?
> > > > >
> > > > > A newly introduced function in include/net/sock.h:
> > > > >
> > > > > static inline int sk_state_load(const struct sock *sk)
> > > > > {
> > > > >         return smp_load_acquire(&sk->sk_state);
> > > > > }
> > > >
> > > > Hmm, maybe we could play a similar trick to READ_ONCE by declaring an
> > > > anonymous union and writing through the non-const member?
> > >
> > > Yes, I think that would work, if you think we need to care about the
> > > case where we read into a structure.
> > >
> > > Can you come up with a patch for that?
> > 
> > Done:
> > 
> >   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.

> You need a static inline function with separate temporaries in each
> branch.

I'm just following the precedent set out by READ_ONCE, since that's
tackling exactly the same problem and appears to be working for people.

Will