Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread

[Thomas Gleixner]

On Wed, 27 Jan 2016, Mathieu Desnoyers wrote:
> +/*
> + * sys_getcpu_cache - setup getcpu cache for caller thread
> + */
> +SYSCALL_DEFINE2(getcpu_cache, int32_t __user **, cpu_cachep, int, flags)
> +{
> +     int32_t __user *cpu_cache;
> +
> +     if (unlikely(flags))
> +             return -EINVAL;
> +     /* Check if cpu_cache is already registered. */
> +     if (current->cpu_cache) {
> +             if (put_user(current->cpu_cache, cpu_cachep))
> +                     return -EFAULT;
> +             return 0;
> +     }

This is really odd. How is the caller supposed to differentiate between:

  1) Get the installed cpucache pointer

  2) Set the cpucache pointer

We really want clearly seperated functionality here.

   getcpu_cache(ptr, GET_CACHEP);
   
and   

   getcpu_cache(ptr, SET_CACHEP);

    Returns -EBUSY if current->cpu_cache is already set, except we allow
    replacing the pointer.  

Thanks,

        tglx