Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of failed SCOND with exponential backoff

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 03, 2015 at 01:50:01PM +0000, Vineet Gupta wrote:
> diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
> index 3dd36c1efee1..c2e012ca4560 100644
> --- a/arch/arc/include/asm/atomic.h
> +++ b/arch/arc/include/asm/atomic.h
> @@ -23,17 +23,50 @@
>  
>  #define atomic_set(v, i) (((v)->counter) = (i))
>  
> +#ifdef CONFIG_ARC_STAR_9000923308
> +
> +#define SCOND_FAIL_RETRY_VAR_DEF                        \
> +    unsigned int delay = 1, tmp;                        \
> +
> +#define SCOND_FAIL_RETRY_ASM                            \
> +    "    bz    4f            \n"                \
> +    "   ; --- scond fail delay ---        \n"                \
> +    "    mov    %[tmp], %[delay]    \n"    /* tmp = delay */    \
> +    "2:     brne.d    %[tmp], 0, 2b        \n"    /* while (tmp != 0) */    \
> +    "    sub    %[tmp], %[tmp], 1    \n"    /* tmp-- */        \
> +    "    asl    %[delay], %[delay], 1    \n"    /* delay *= 2 */    \

You forgot the overflow test .. :-)

> +    "    b    1b            \n"    /* start over */    \
> +    "4: ; --- success ---            \n"                \
> +
> +#define SCOND_FAIL_RETRY_VARS                            \
> +      ,[delay] "+&r" (delay),[tmp] "=&r"    (tmp)                \
> +
> +#else    /* !CONFIG_ARC_STAR_9000923308 */
> +
> +#define SCOND_FAIL_RETRY_VAR_DEF
> +
> +#define SCOND_FAIL_RETRY_ASM                            \
> +    "    bnz     1b            \n"                \
> +
> +#define SCOND_FAIL_RETRY_VARS
> +
> +#endif

But yes, much better.