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

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Message-ID: <671969438.6129.1453915918933.JavaMail.zimbra@efficios.com>
Date:	Wed, 27 Jan 2016 17:31:58 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	linux-api <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>,
	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 Jan 27, 2016, at 12:22 PM, Thomas Gleixner tglx@...utronix.de wrote:

> 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.

Sounds fair. What is the recommended typing for "ptr" then ?
uint32_t ** or uint32_t * ?

It would be expected to pass a "uint32_t *" for the set
operation, but the "get" operation requires a "uint32_t **".

Also, I'd be tempted to put the GET/SET operation selector as
a first parameter.

Thanks,

Mathieu

> 
> Thanks,
> 
> 	tglx

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com