I have shared memory segment created in kernel using mmap. I need to access this mapped memory from both kernel and user space. What mechanism should I use to protect the memory from concurrent access ? 
I want to have something like:

Kernel module:

```
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/debugfs.h>
#include <linux/slab.h>
#include <linux/mm.h> 

#ifndef VM_RESERVED
# define  VM_RESERVED   (VM_DONTEXPAND | VM_DONTDUMP)
#endif

struct dentry  *file;

struct mmap_info
{
    char *data;            
    int reference;      
};

void mmap_open(struct vm_area_struct *vma)
{
    struct mmap_info *info = (struct mmap_info *)vma->vm_private_data;
    info->reference++;
}

void mmap_close(struct vm_area_struct *vma)
{
    struct mmap_info *info = (struct mmap_info *)vma->vm_private_data;
    info->reference--;
}

static int mmap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
    struct page *page;
    struct mmap_info *info;    

    info = (struct mmap_info *)vma->vm_private_data;
    if (!info->data)
    {
        printk("No data\n");
        return 0;    
    }
    page = virt_to_page(info->data);    
    get_page(page);
    vmf->page = page;            
    return 0;
}

struct vm_operations_struct mmap_vm_ops =
{
    .open =     mmap_open,
    .close =    mmap_close,
    .fault =    mmap_fault,    
};

int op_mmap(struct file *filp, struct vm_area_struct *vma)
{
    vma->vm_ops = &mmap_vm_ops;
    vma->vm_flags |= VM_RESERVED;    
    vma->vm_private_data = filp->private_data;
    mmap_open(vma);
    return 0;
}

int mmapfop_close(struct inode *inode, struct file *filp)
{
    struct mmap_info *info = filp->private_data;
    free_page((unsigned long)info->data);
    kfree(info);
    filp->private_data = NULL;
    return 0;
}

int mmapfop_open(struct inode *inode, struct file *filp)
{
    struct mmap_info *info = kmalloc(sizeof(struct mmap_info), GFP_KERNEL);    
    info->data = (char *)get_zeroed_page(GFP_KERNEL);
    memcpy(info->data, "hello from kernel this is file: ", 32);
    memcpy(info->data + 32, filp->f_dentry->d_name.name, strlen(filp->f_dentry->d_name.name));
    /* assign this info struct to the file */
    filp->private_data = info;
    return 0;
}

static const struct file_operations mmap_fops = {
    .open = mmapfop_open,
    .release = mmapfop_close,
    .mmap = op_mmap,
};

 static int __init mmapexample_module_init(void)
{
    file = debugfs_create_file("mmap_example", 0644, NULL, NULL, &mmap_fops);
    return 0;
}

static void __exit mmapexample_module_exit(void)
{
    debugfs_remove(file);
}

module_init(mmapexample_module_init);
module_exit(mmapexample_module_exit);
MODULE_LICENSE("GPL");

```

User space:

```
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <sys/mman.h>

#define PAGE_SIZE     4096

int main ( int argc, char **argv )
{
    int configfd;
    char * address = NULL;

    configfd = open("/sys/kernel/debug/mmap_example", O_RDWR);
    if(configfd < 0)
    {
        perror("Open call failed");
        return -1;
    }

    address = mmap(NULL, PAGE_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, configfd, 0);
    if (address == MAP_FAILED)
    {
        perror("mmap operation failed");
        return -1;
    }
    printf("Initial message: %s\n", address);
    memcpy(address + 11 , "*user*", 6);
    printf("Changed message: %s\n", address);
    close(configfd);    
    return 0;
}

```

but with locks.