[PATCH V4] arm64: percpu: Implement this_cpu operations

[Will Deacon]

Hi Steve,

On Fri, Nov 14, 2014 at 03:03:33PM +0000, Steve Capper wrote:
> The generic this_cpu operations disable interrupts to ensure that the
> requested operation is protected from pre-emption. For arm64, this is
> overkill and can hurt throughput and latency.
> 
> This patch provides arm64 specific implementations for the this_cpu
> operations. Rather than disable interrupts, we use the exclusive
> monitor or atomic operations as appropriate.
> 
> The following operations are implemented: add, add_return, and, or,
> read, write, xchg. We also wire up a cmpxchg implementation from
> cmpxchg.h.
> 
> Testing was performed using the percpu_test module and hackbench on a
> Juno board running 3.18-rc4.

Looks good. I notice that this change drops the compiler barriers too,
which we used to get via local_irq_{enable/disable}. I *think* that's fine
(at least, I couldn't find a place that breaks due to that) but it would be
nice to know that it was deliberate :)

> +static inline void __percpu_write(void *ptr, unsigned long val, int size)
> +{
> +	switch (size) {
> +	case 1:
> +		ACCESS_ONCE(*((u8 *)ptr)) = (u8) val;
> +		break;
> +	case 2:
> +		ACCESS_ONCE(*((u16 *)ptr)) = (u16) val;
> +		break;
> +	case 4:
> +		ACCESS_ONCE(*((u32 *)ptr)) = (u32) val;
> +		break;
> +	case 8:
> +		ACCESS_ONCE(*((u64 *)ptr)) = (u64) val;
> +		break;

I think you've gone a bit overboard with brackets and spacing here. Can't
you just do something like:

  ACCESS_ONCE(*(u64 *)ptr) = (u64)val;

Anyway:

  Reviewed-by: Will Deacon <will.deacon at arm.com>

Will