From: Arnd Bergmann
> Sent: 17 November 2015 19:17
> 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.
How are you going to find the right type/size for the non-const member of the union?
If the type/size is fixed then why use typeof()?
If the variable being read (sk->sk_state above) might only be 8 or 16 bits then
you might be able to use sizeof() to select between code paths.
David
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