Dynamically creating variable in Kernel Module

Question

I am planning to use kthread_run API in my kernel module.
As kthread_run, returns a struct task_struct *, which i want to store in a variable global across my module.
However, i want one thread each for a cpu and i obtain no of cpus using num_online_cpus.
However, when i write following code:

    //outside init_module function
    int cpus = num_online_cpus();
    static struct task_struct *my_tasks[cpus];  

    static int __init init_module(){
           for(int j = 0; j < cpus; ++j){
                  my_tasks[j] = kthread_run(...);  
           }  
    }

However, i get following error on this:
error: variably modified ‘tasks’ at file scope
How can i achieve this ???

Your loop variable is j, but the array subscript is i? Anyway, use kmalloc — Eugene Sh.
– Eugene Sh., Commented Mar 5, 2015 at 18:22
I think the problem is declaring *my_tasks[cpus]. When using global variables, which by the way is usually a bad choice, the sizes of your arrays must be compile time constants. — Milind Dumbare
– Milind Dumbare, Commented Mar 5, 2015 at 19:58
@Miline: Thats exactly the point. But how can i declare the array with exact size as num_of_cpus ? — Vikas Raturi
– Vikas Raturi, Commented Mar 6, 2015 at 6:59
Dont create array. Create dynamic linked list of items with each node as task_struct — Milind Dumbare
– Milind Dumbare, Commented Mar 6, 2015 at 10:01
@Vikas, You have the answer already below. Try to invest your time into your problem. — 0andriy
– 0andriy, Commented Mar 6, 2015 at 17:56

John · Accepted Answer · 2015-03-05 19:02:44Z

1

If your variables are actually one per cpu, you might want to use the per_cpu macros. The gist is, you declare such a variable with:

DEFINE_PER_CPU(struct task_struct, my_tasks);

and then access the variable using

get_cpu_var(my_tasks).foo = bar;

You can get more info at See http://www.makelinux.net/ldd3/chp-8-sect-5 (or percpu.h) for more details.

answered Mar 5, 2015 at 19:02

John

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2 Comments

Vikas Raturi Over a year ago

That's good but not usable in my situation. I need to access all variables together in main process and then create threads for each.

John Over a year ago

You can access the variable of another cpu using per_cpu. (i.e.: per_cpu(my_tasks, other_cpu).foo = bar).

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