linux-kernel - Re: [RFC PATCH] use snprintf instead of sprintf in rcu_torture

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Date:	Thu, 19 Jun 2014 20:13:22 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Joe Perches <joe@...ches.com>, LKML <linux-kernel@...r.kernel.org>,
	romanov.arya@...il.com, Josh Triplett <josh@...htriplett.org>
Subject: Re: [RFC PATCH] use snprintf instead of sprintf in rcu_torture_printk

On 06/19/2014 07:49 PM, Paul E. McKenney wrote:
> On Thu, Jun 19, 2014 at 07:24:48PM -0400, Pranith Kumar wrote:
>> (dropping some CCs)
>>
>> On 06/19/2014 05:00 PM, Paul E. McKenney wrote:
>>> On Wed, Jun 18, 2014 at 12:49:42PM -0700, Joe Perches wrote:
>>>>
>>>> I believe the function doesn't work well.
>>>>
>>>> static void
>>>> rcu_torture_stats_print(void)
>>>> {
>>>> 	int size = nr_cpu_ids * 200 + 8192;
>>>> 	char *buf;
>>>>
>>>> 	buf = kmalloc(size, GFP_KERNEL);
>>>> 	if (!buf) {
>>>> 		pr_err("rcu-torture: Out of memory, need: %d\n", size);
>>>> 		return;
>>>> 	}
>>>> 	rcu_torture_printk(buf);
>>>> 	pr_alert("%s", buf);
>>>> 	kfree(buf);
>>>> }
>>>>
>>>> rcu_torture_printk simply fills buf
>>>>
>>>> btw: I believe the arguments should pass size and
>>>>      rcu_torture_printk should use snprintf/size
>>>>
>>>> but all printks are limited to a maximum of 1024
>>>> bytes so the large allocation is senseless and
>>>> would even if it worked, would likely need to be
>>>> vmalloc/vfree
>>>
>>> Fair point!
>>>
>>> Pranith, Romanov, if you would like part of RCU that is less touchy
>>> about random hacking, this would be a good place to start.  Scripts in
>>> tools/testing/selftests/rcutorture/bin do care about some of the format,
>>> but the variable-length portion generated by cur_ops->stats() is as far
>>> as I know only parsed by human eyes.
>>>
>>
>> Here is a first run of the change. Please let me know if I am totally off. RFC. :)
> 
> Thank you for taking this on!

You are most welcome :)

> 
>> Three things on Todo list:
>>
>> * We need to check that we are using less than the allocated size of the buffer (used > size). (we are allocating a big buffer, so not sure if necessary)
>> * Need to check with the scripts if they are working.
>> * I used a loop for pr_alert(). I am not sure this is right, there should be a better way for printing large buffers
>>
>> If the overall structure is ok I will go ahead and check how the scripts are handling these changes. 
> 
> One other thing...  Convince this function (and a few others that it
> calls) that the system really has 4096 CPUs, run this code, and see what
> actually happens both before and after.  Just to get a bit of practice
> mixed in with the theory.  ;-)
> 

OK. I think to do this I need to use 4096 instead of nr_cpu_ids. I will try this and see how it goes :)

--
Pranith
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