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

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Message-ID: <alpine.DEB.2.11.1601271813290.3886@nanos>
Date:	Wed, 27 Jan 2016 18:22:04 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number
 of running thread

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