From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number
 of running thread
Date: Wed, 27 Jan 2016 18:22:04 +0100 (CET)
Message-ID: <alpine.DEB.2.11.1601271813290.3886@nanos>
References: <1453913683-28915-1-git-send-email-mathieu.desnoyers@efficios.com> <1453913683-28915-2-git-send-email-mathieu.desnoyers@efficios.com>
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To: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Cc: Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>, Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>, Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>, Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>, Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
List-Id: linux-api@vger.kernel.org

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