linux-kernel - whats wrong with my code ? trying to lock and iterate through read tasks in cfs rq

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Message-ID: <CAN2J1_ObiGJnwdcEfEmrX4tyX+xEaf_Xz4cc-wmC+aCaeWr4MA@mail.gmail.com>
Date:	Sat, 19 Mar 2016 01:06:26 +0200
From:	nad gam <nadavkernel@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: whats wrong with my code ? trying to lock and iterate through read
 tasks in cfs rq

here is the code of system call I implemented trying to iterate
through cfs tasks of cpu_num rq and retrieve their pids  :

#include <linux/kernel.h>
#include <linux/sched.h>
#include "/home/user/linux-3.14.1/kernel/sched/sched.h"
#include <linux/slab.h>
#include <asm/uaccess.h>

asmlinkage long sys_get_tasks_from_rq(int cpu_num,void* dst)
{
        struct rq * my_rq;

        my_rq=cpu_rq(cpu_num);
        int* pids = (int*) kmalloc(sizeof(int)*my_rq->nr_running,GFP_KERNEL);
        struct task_struct *p, *n;
        int i=0;
        raw_spin_lock_irq(&my_rq->lock);
        list_for_each_entry_safe(p, n, &my_rq->cfs_tasks, se.group_node){
                pids[i]=task_pid_vnr(p);
                i++;
        }

        raw_spin_unlock_irq(&my_rq->lock);

        copy_to_user(dst,pids,300);
        return 0;
}


when I try to use it in user space I get segmenation fault :

int main(int argc,char** argv){
        int cpuNum = atoi(argv[1]);
        int pids[300];
        syscall(319,cpuNum,pids);
        int i;
        for (i=0;i<10;i++){
                printf("%d \t",pids[i]);
        }
}

So I guess Im doing something wrong :/ Could some plz direct me for
where my problem is? Thanks !