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