[v8,12/12] powerpc: rewrite local_t using soft_irq

[Steven Roberts]

On Mon, Apr 17, 2017 at 06:47:37AM +0530, Madhavan Srinivasan wrote:
> Local atomic operations are fast and highly reentrant per CPU counters.
> Used for percpu variable updates. Local atomic operations only guarantee
> variable modification atomicity wrt the CPU which owns the data and
> these needs to be executed in a preemption safe way.
>
> Here is the design of this patch. Since local_* operations
> are only need to be atomic to interrupts (IIUC), we have two options.
> Either replay the "op" if interrupted or replay the interrupt after
> the "op". Initial patchset posted was based on implementing local_* operation
> based on CR5 which replay's the "op". Patchset had issues in case of
> rewinding the address pointor from an array. This make the slow path
> really slow. Since CR5 based implementation proposed using __ex_table to find
> the rewind addressr, this rasied concerns about size of __ex_table and vmlinux.
>
> https://lists.ozlabs.org/pipermail/linuxppc-dev/2014-December/123115.html
>
> +static __inline__ int local_add_unless(local_t *l, long a, long u)
> +{
> +     long t;
> +     unsigned long flags;
> +
> +     powerpc_local_irq_pmu_save(flags);
> +     __asm__ __volatile__ (
> +     PPC_LL" %0,0(%1)\n\
> +     cmpw    0,%0,%3 \n\

Loading a long work here and comparing a single word.  Replace cmpw with PPC_LCMP.
> +     beq-    2f \n\
> +     add     %0,%2,%0 \n"
> +     PPC_STL" %0,0(%1) \n"
> +"       subf %0,%2,%0 \n\
> +2:"
> +        : "=&r" (t)
> +        : "r" (&(l->a.counter)), "r" (a), "r" (u)
> +        : "cc", "memory");
> +        powerpc_local_irq_pmu_restore(flags);
> +
> +     return t != u;
> +}
> +
> +#define local_inc_not_zero(l) local_add_unless((l), 1, 0)
> +
> +#define local_sub_and_test(a, l)     (local_sub_return((a), (l)) == 0)
> +#define local_dec_and_test(l)                (local_dec_return((l)) == 0)
> +
> +/*
> + * Atomically test *l and decrement if it is greater than 0.
> + * The function returns the old value of *l minus 1.
> + */
> +static __inline__ long local_dec_if_positive(local_t *l)
> +{
> +     long t;
> +     unsigned long flags;
> +
> +     powerpc_local_irq_pmu_save(flags);
> +     __asm__ __volatile__(
> +     PPC_LL" %0,0(%1)\n\
> +     cmpwi   %0,1\n\

Same issue here.  Replace cmpwi with PPC_LCMPI.

> +     addi    %0,%0,-1\n\
> +     blt-    2f\n"
> +     PPC_STL "%0,0(%1)\n"
> +     "\n\
> +2:"  : "=&b" (t)
> +     : "r" (&(l->a.counter))
> +     : "cc", "memory");
> +     powerpc_local_irq_pmu_restore(flags);
> +
> +     return t;
> +}
> +