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Date: Wed, 27 Jan 2016 18:22:04 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: Paul Turner <pjt@google.com>, Andrew Hunter <ahh@google.com>,
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Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number
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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